igb: Update MTU so that it is always at least a standard frame size

This change makes it so that we limit the lower bound for max_frame_size to
the size of a standard Ethernet frame.  This allows for feature parity with
other Intel based drivers such as ixgbe.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>