The control step values reported by the device are used as a divisor
unchecked, which can result in a division by zero.

Check the step value and make it 1 when null.
Signed-off-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>