sky2: hand receive DMA mapping failures

If receive buffer mapping failed, then it was possible to get
stuck with unmapped receive buffer in DMA ring.

This would be an extremely rare condition because the driver had just
released the map for the last receive so it should be able to get
another map again (in soft-irq).

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>