[media] atomisp: unify sh_css_hmm_buffer_record_acquire

The ISP2401 version of this function returns a pointer to the buffer, whilst
the ISP2400 version returns a boolean if a slot is found. We can trivially
unify the code to use the ISP2401 version.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>