bnxt_en: Support max-mtu with VF-reps

While a VF is configured with a bigger mtu (> 1500), any packets that
are punted to the VF-rep (slow-path) get dropped by OVS kernel-datapath
with the following message: "dropped over-mtu packet". Fix this by
returning the max-mtu value for a VF-rep derived from its corresponding VF.
VF-rep's mtu can be changed using 'ip' command as shown in this example:

	$ ip link set bnxt0_pf0vf0 mtu 9000

Signed-off-by: Sriharsha Basavapatna <sriharsha.basavapatna@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>