media: cx25840: fix a possible divide by zero in set_fmt callback

If set_fmt callback is called with format->width or format->height set to
zero and HACTIVE_CNT or VACTIVE_CNT bits (respectively) in chip are zero
we will divide by zero later in this callback when we try to calculate
HSC or VSC values.

Fix this by explicitly rejecting these values.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>