enic: handle mtu change for vf properly

When driver gets notification for mtu change, driver does not handle it for
all RQs. It handles only RQ[0].

Fix is to use enic_change_mtu() interface to change mtu for vf.

Signed-off-by: Govindarajulu Varadarajan <gvaradar@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>