caif: fix a signedness bug in cfpkt_iterate()

The cfpkt_iterate() function can return -EPROTO on error, but the
function is a u16 so the negative value gets truncated to a positive
unsigned short.  This causes a static checker warning.

The only caller which might care is cffrml_receive(), when it's checking
the frame checksum.  I modified cffrml_receive() so that it never says
-EPROTO is a valid checksum.

Also this isn't ever going to be inlined so I removed the "inline".

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>