V4L/DVB (3568i): adv7175: Drop unused register cache

Drop the adv7175 register cache, as it is only written to and never read back
from.  This saves 128 bytes of memory and slightly speeds up the register
writes.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>