bna: add missing per queue ethtool stat

Commit ba5ca784 "bna: check for dma mapping errors" added besides other
things a statistic that counts number of DMA buffer mapping failures
per each Rx queue. This counter is not included in ethtool stats output.

Fixes: ba5ca784 "bna: check for dma mapping errors"
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>