V4L/DVB (8672): gspca: Big rewrite of spca561.

Bug register/value inversions in USB exchanges.
Exposure and gain controls added for rev 12a.
Separate the functions and controls of the revisions 12a and 72a.
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>