V4L/DVB (8193): gspca: Input buffer may be changed on reg write.

Done for conex, etoms, pac7311, sonixj, t613 and tv8532.
Code cleanup for some other subdrivers.

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>