net: dsa: tag_qca: fix wrong MGMT_DATA2 size

It was discovered that MGMT_DATA2 can contain up to 28 bytes of data
instead of the 12 bytes written in the Documentation by accounting the
limit of 16 bytes declared in Documentation subtracting the first 4 byte
in the packet header.

Update the define with the real world value.

Tested-by: Ronald Wahl <ronald.wahl@raritan.com>
Fixes: c2ee8181 ("net: dsa: tag_qca: add define for handling mgmt Ethernet packet")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Cc: stable@vger.kernel.org # v5.18+
Signed-off-by: David S. Miller <davem@davemloft.net>