aqc111: cleanup mtu related logic

Original fix b8b27752 was done under impression that invalid data
could be written for mtu configuration higher that 16334.

But the high limit will anyway be rejected my max_mtu check in caller.
Thus, make the code cleaner and allow it doing the configuration without
checking for maximum mtu value.

Fixes: b8b27752 ("aqc111: fix endianness issue in aqc111_change_mtu")
Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>