liquidio: fix ndo_change_mtu to always return correct status to the caller

In a scenario where the command queued to firmware get dropped or times
out, MTU change from host will not propagate to firmware. So, it is
required for host driver to wait for response from firmware or timeout
and then return correct status to caller of ndo_change_mtu.

Also moved the common code for MTU change from PF and VF driver files to
common file lio_core.c

Signed-off-by: Veerasenareddy Burru <veerasenareddy.burru@cavium.com>
Signed-off-by: Felix Manlunas <felix.manlunas@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>