defxx: Discard DMA maps on buffer deallocation

Prearranged receive DMA bounce buffer mappings are not released in the
card reboot/shutdown path.  That does not affect frame reception, but
probably explains the random segmentation fault I observed the other day
on interface shutdown.  Card is rebooted as required by the spec in the
process of ring fault recovery when a PC Trace signal has been received.

This change fixes the problem in an obvious manner.

Reported-by: Robert Coerver <Robert.Coerver@ll.mit.edu>
Tested-by: Robert Coerver <Robert.Coerver@ll.mit.edu>
Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>