i40e: clamp jumbo frame size

The driver was allowing the user to set larger size MTU
than the hardware was being configured to support.

The driver was already using VLAN_HLEN when setting the
hardware max receivable frame size, so just add it to the
netdev MTU set entry point as well.

Change-ID: Ie20e2a35d04f8c411253e255bea79ca69aaeaea3
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>