V4L/DVB (10761): cx18: Change log lines for internal subdevs and fix tveeprom reads

Give messages originating from internal subdevs a header using the subdev's
name.  Fixed an uninitialized variable problem with reading the EEPROM, noticed
from log output.  Got rid of the unused cx18_av_exit() function.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>