mlxsw: core_acl_flex_keys: Change SRC_SYS_PORT flex key element size

The SRC_SYS_PORT is passed as 8 bit value down to hw anyway, so cap it
in the driver as well. Also, in Spectrum-2 the FW iface for SRC_SYS_PORT
is only 8 bits, so prepare for it.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>