fjes: Fix unnecessary spinlock_irqsave

commit-bd5a2569 introduces a deadlock bug in fjes_change_mtu().
This spin_lock_irqsave() is obviously unnecessary.

This patch eliminates unnecessary spin_lock_irqsave() in
fjes_change_mtu()

Signed-off-by: Taku Izumi <izumi.taku@jp.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>