V4L/DVB (11789): xc5000: check xc5000_readreg return value for XC_RESULT_SUCCESS

Make return value checking for calls to i2c routines explicit.

Signed-off-by: Devin Heitmueller <devin.heitmueller@gmail.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>