media: xc5000: better handle I2C error messages

As warned by smatch, there are several places where the I2C
transfer may fail, leading into inconsistent behavior:

	drivers/media/tuners/xc5000.c:689 xc_debug_dump() error: uninitialized symbol 'regval'.
	drivers/media/tuners/xc5000.c:841 xc5000_is_firmware_loaded() error: uninitialized symbol 'id'.
	drivers/media/tuners/xc5000.c:939 xc5000_set_tv_freq() error: uninitialized symbol 'pll_lock_status'.
	drivers/media/tuners/xc5000.c:1195 xc_load_fw_and_init_tuner() error: uninitialized symbol 'pll_lock_status'.

Handle the return codes from the I2C transfer, in order to
address those issues.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>