NFC: fdp: Fix a signedness bug in fdp_nci_send_patch()

The nci_conn_max_data_pkt_payload_size() function sometimes returns
-EPROTO so "max_size" needs to be signed for the error handling to
work.  We can make "payload_size" an int as well.

Fixes: a06347c0

 ("NFC: Add Intel Fields Peak NFC solution driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>