bgmac: configure MTU and add support for frames beyond 8192 byte size

Change DMA descriptor length to handle jumbo frames beyond 8192 bytes.
Also update jumbo frame max size to include FCS, the DMA packet length
received includes FCS.

Signed-off-by: Murali Krishna Policharla <murali.policharla@broadcom.com>
Reviewed-by: Arun Parameswaran <arun.parameswaran@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>