net: axienet: Handle jumbo frames for lesser frame sizes

In the current implementation, jumbo frames are supported only
for the frame sizes > 16K. This patch corrects this logic to
handle jumbo frames for lesser frame sizes (< 16K) ensuring jumbo frame
MTU is within the limit of max frame size configured in the h/w
design.

Signed-off-by: Srikanth Thokala <sthokal@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>