doc: mysql: declare MAC protocol columns unsigned

doc: mysql: declare MAC protocol columns unsigned

By default, MySQL smallints are signed.  This causes problems inserting packets
for ethertypes above 0x7fff, such as IPv6 (0x86dd):

  MariaDB [ulogd]> SELECT INSERT_PACKET_FULL(...,'f4:7b:09:41:7a:71','f0:2f:74:4e:b2:f3',34525,0,NULL,NULL,NULL);
                                                                                         ^^^^^

which fails as follows:

  ERROR 1264 (22003): Out of range value for column 'mac_protocol' at row 1

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>