[media] mt9v032: fix hblank calculation

Since (min_row_time - crop->width) can be negative, we have to do a signed
comparison here. Otherwise max_t casts the negative value to unsigned int
and sets min_hblank to that invalid value.

Cc: stable@vger.kernel.org
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>