usb: gadget: f_fs: Fix kernel panic if use_os_string not set

If c->cdev->use_os_string flag is not set,
don't need to invoke ffs_do_os_descs() in _ffs_func_bind.
So uninitialized ext_compat_id pointer won't be accessed by
__ffs_func_bind_do_os_desc to cause kernel panic.

Change-Id: I7a08d683bb068c8015675a0676c4a092272e5e0b
Signed-off-by: Jim Lin <jilin@nvidia.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Git-commit: c6010c8b4d2c6e75853ca63c602c9af56fcbead5
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Sai Krishna Juturi <jsaikrishna@codeaurora.org>