net: smc91c111: flush packets on RCR register changes

The SOFT_RST or RXEN in the control register can be used as a condition
to unblock the net layer via can_receive(). So check for possible
flushes on RCR changes. This will drop all pending packets on soft
reset or disable which is the functional intent of the can_receive()
logic.

Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Tested-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Message-id: b114d4c96f4afbdaa15f1361d9c07e3021755915.1441873621.git.crosthwaite.peter@gmail.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>