media: atomisp: check before deference asd variable

The asd->isp was referenced before checking if asd is not
NULL.

This fixes this warning:

	../drivers/staging/media/atomisp/pci/atomisp_cmd.c:5548 atomisp_set_fmt_to_snr() warn: variable dereferenced before check 'asd' (see line 5540)

While here, avoid getting the pipe pointer twice.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>