V4L/DVB (13419): gspca - pac7302: Add debug register write interface.

Add debug register write interface to pac7302 to be able to set
for example the edge detect mode (bit 2 register 0x55) or the
test pattern (bit 0..3, register 0x72) and test overlay (bit 4,
register 0x72) from the user space. Only write of register
page 0 is supported by this patch.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>