enic: do not call enic_change_mtu in enic_probe

In commit ab123fe0 ("enic: handle mtu change for vf properly")
ASSERT_RTNL() is added to _enic_change_mtu() to prevent it from being
called without rtnl held. enic_probe() calls enic_change_mtu()
without rtnl held. At this point netdev is not registered yet.
Remove call to enic_change_mtu and assign the mtu to netdev->mtu.

Fixes: ab123fe0 ("enic: handle mtu change for vf properly")
Signed-off-by: Govindarajulu Varadarajan <gvaradar@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>