[media] usb/cpia2_core: clean up a min_t() cast

It makes sense to make the min_t() cast unsigned here since we don't
really want negative sizes.  Making it signed causes a static checker
warning in Smatch.  Smatch knows "fw->size - i" is positive but it
doesn't know that fw->size is less than INT_MAX so in theory casting it
to int might lead to a negative.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>