igb: Allow asymmetric configuration of MTU versus Rx frame size

Since the igb driver is using page based receive there is no point in
limiting the Rx capabilities of the device.  The driver can receive 9K
jumbo frames at all times.  The only changes needed due to MTU changes are
updates for the FIFO sizes and flow-control watermarks.

Update the maximum frame size to reflect the 9.5K limitation of the
hardware, and replace all instances of max_frame_size with
MAX_JUMBO_FRAME_SIZE when referring to an Rx FIFO or frame.

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