V4L/DVB (11788): xc5000: cleanup i2c write routines

Cleanup the i2c write routine, getting rid of a passthrough function with only
one caller

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>