net: fec: quiesce packet processing when taking link down in fec_enet_adjust_link()

When the link goes down, the adjust_link method will be called, but
there is no synchronisation to ensure that we won't be processing some
last remaining packets via the NAPI handlers while performing a reset of
the device.

Add the necessary synchronisation to ensure that packet processing
is complete before we stop and reset the FEC.

Acked-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>