net:ethernet:aquantia: Remove adapter re-opening when MTU changed.

Closing/opening the adapter is not needed at all.
The new MTU settings take effect immediately.

Fixes: 97bde5c4

 ("net: ethernet: aquantia: Support for NIC-specific code")

Signed-off-by: Pavel Belous <pavel.belous@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>