V4L/DVB (5476): Fix gpiomux array size

there were several "magic" for loops, addressing gpiomux array size (4).
Adrian Bunk showed that one of the loops were wrong, going from 0 to 4.

This patch provides the right fix for this trouble, by using ARRAY_SIZE
on all places where we have a for loop using gpiomux.

Thanks to: Adrian Bunk <bunk@stusta.de> for pointing me about this trouble.
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>