NFC: fdp: fix signed less or equal zero check in u8 max_size

The u8 variable max_size is being assigned a return value from the
call to nci_conn_max_data_pkt_payload_size that can return a -ve
error return. Since max_size is a u8, the -ve check for the error will
always be false. Fix this by making max_size an int type.

Detected using Coccinelle:
drivers/nfc/fdp/fdp.c:208:5-13: WARNING: Unsigned expression compared
with zero: max_size <= 0

Fixes: a06347c04c13 ("NFC: Add Intel Fields Peak NFC solution driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>