V4L/DVB (6903): mt312: CodingStyle fix

Fixes all occurences of assignment in if

checkpatch marks them as ERROR.

Signed-off-by: Matthias Schwarzott <zzam@gentoo.org>
Reviewed-by: Andreas Oberritter <obi@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>