net: macb: Correct the MID field length value

The latest spec "I-IPA01-0266-USR Rev 10" limit the MID field length
to 12 bit value. For previous versions it is 16 bit value.

This change will not break the backward compatibility as the latest
ID value is 7 and with in the 12 bit value limit.

Signed-off-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>