[media] cx231xx: handle errors at read_eeprom()

Fix the following warnings:
	drivers/media/usb/cx231xx/cx231xx-cards.c: In function 'read_eeprom':
	drivers/media/usb/cx231xx/cx231xx-cards.c:979:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>