[qca-nss-clients] Allow setting MTU based on underlying physical gmac interface's MTU

eth_change_mtu() does not allow setting MTU larger than 1500. Now we look for
the underlying gmac's MTU to decide if a new MTU value is valid or not

Change-Id: Ibcd9a3b352eb0f181ce709dd8cf7f6d493bf8fac
Signed-off-by: Stephen Wang <wstephen@codeaurora.org>
2 files changed