staging: atomisp: simplify if statement in atomisp_get_sensor_fps()

If v4l2_subdev_call() gets the global frame interval values,
it returned 0 and it could be checked whether numerator is zero or not.

If the numerator is not zero, the fps could be calculated in this function.
If not, it just returns 0.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>