net: fec: quiesce packet processing before changing features

Changing the features (receive checksumming) requires the hardware to be
reprogrammed, and also changes the checks in the receive packet
processing.

The current implementation has a race - fec_set_features() changes the
flags which alter the receive packet processing while the adapter is
active, and potentially receiving frames.  Only after we've modified
the software flag do we shutdown and reconfigure the hardware.

This can lead to packets being received and marked with a valid checksum
(via CHECKSUM_UNNECESSARY) when the hardware checksum validation has not
yet been enabled.

We must quiesce the device, then change the software configuration for
this feature, and then resume the device if it was previously running.

The resulting code structure also allows us to add other configuration
features in this path without having to quiesce and resume the network
interface and device.

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>