V4L/DVB (7608): em28xx-dvb: Some cleanups and fixes

em28xx-dvb.c:
 - Remove unneeded xc3028_ctrl structure.  The driver automatically preserves
   the previous value

tuner-xc2028.c:
 - Make the return type for xc2028_get_reg signed, since all of the callers
   are looking for "< 0" to detect errors.

Signed-off-by: Devin Heitmueller <devin.heitmueller@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>