media: atomisp: fix call to g_frame_interval

The media core has now a check if fi->pad is bigger than zero
or bigger than sd->entity.num_pads, if the media controller
is defined.

This causes a call to g_frame_interval to return -EINVAL.

Fix it by first cleaning up the struct.

Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>