media: vicodec: change variable name for the return value of v4l2_fwht_encode

v4l2_fwht_encode returns either an error code on
failure or the size of the compressed frame on
success. So change the var assigned to it from
'ret' to 'comp_sz_or_errcode' to clarify that.

Signed-off-by: Dafna Hirschfeld <dafna3@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>