V4L/DVB (5212): Pvrusb2: Be more forgiving about encoder firmware size

The pvrusb2 driver previously rejected encoder firmware whose size was
not a multiple of 8192.  But this is a false check because it's
possible to find cx23416 firmware whose size doesn't conform to this
limit.  So change the firmware loader implementation to be more
forgiving of the image size.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>