[media] staging: atomisp: Set step to 0 for mt9m114 menu control

menu controls are not allowed to have a step size, set step to 0 to
fix an oops from the WARN_ON in v4l2_ctrl_new_custom() triggering
because of this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>