[media] exynos4-is: Prevent NULL pointer dereference when firmware isn't loaded

Ensure the firmware isn't accessed in the driver when the firmware loading
routine has not completed. This fixes a potential kernel crash:
[   96.510000] Unable to handle kernel NULL pointer dereference at virtual address 00000000
[   96.520000] pgd = ee604000
[   96.520000] [00000000] *pgd=6e947831, *pte=00000000, *ppte=00000000
[   96.530000] Internal error: Oops: 17 [#1] PREEMPT SMP ARM
[   96.530000] Modules linked in:
[   96.530000] CPU: 2 PID: 2787 Comm: camera_test Not tainted 3.10.0-rc1-00269-gcdbde37-dirty #2158
[   96.545000] task: ee42e400 ti: edfcc000 task.ti: edfcc000
[   96.545000] PC is at fimc_is_start_firmware+0x14/0x94
[   96.545000] LR is at fimc_isp_subdev_s_power+0x13c/0x1f8
	...
[   96.745000] [<c03e0354>] (fimc_is_start_firmware+0x14/0x94) from [<c03e1cc4>] (fimc_isp_subdev_s_power+0x13c/0x1f8)
[   96.745000] [<c03e1cc4>] (fimc_isp_subdev_s_power+0x13c/0x1f8) from [<c03ed088>] (__subdev_set_power+0x70/0x84)
[   96.745000] [<c03ed088>] (__subdev_set_power+0x70/0x84) from [<c03ed164>] (fimc_pipeline_s_power+0xc8/0x164)
[   96.745000] [<c03ed164>] (fimc_pipeline_s_power+0xc8/0x164) from [<c03ee2b8>] (__fimc_pipeline_open+0x90/0x268)
[   96.745000] [<c03ee2b8>] (__fimc_pipeline_open+0x90/0x268) from [<c03ec5f0>] (fimc_capture_open+0xe4/0x1ec)
[   96.745000] [<c03ec5f0>] (fimc_capture_open+0xe4/0x1ec) from [<c03c5560>] (v4l2_open+0xa8/0xe4)
[   96.745000] [<c03c5560>] (v4l2_open+0xa8/0xe4) from [<c0112900>] (chrdev_open+0x9c/0x158)
[   96.745000] [<c0112900>] (chrdev_open+0x9c/0x158) from [<c010d3e0>] (do_dentry_open+0x1f4/0x27c)
[   96.745000] [<c010d3e0>] (do_dentry_open+0x1f4/0x27c) from [<c010d558>] (finish_open+0x34/0x50)
[   96.745000] [<c010d558>] (finish_open+0x34/0x50) from [<c011bea0>] (do_last+0x59c/0xbcc)
[   96.745000] [<c011bea0>] (do_last+0x59c/0xbcc) from [<c011c580>] (path_openat+0xb0/0x484)
[   96.745000] [<c011c580>] (path_openat+0xb0/0x484) from [<c011ca58>] (do_filp_open+0x30/0x84)
[   96.745000] [<c011ca58>] (do_filp_open+0x30/0x84) from [<c010d060>] (do_sys_open+0xe8/0x170)
[   96.745000] [<c010d060>] (do_sys_open+0xe8/0x170) from [<c000f040>] (ret_fast_syscall+0x0/0x30)

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>