atl1e: unmap partially mapped skb on dma error and free skb

Ben Hutchings pointed out that my recent update to atl1e
in commit 352900b5
("atl1e: fix dma mapping warnings") was missing a bit of code.

Specifically it reset the hardware tx ring to its origional state when
we hit a dma error, but didn't unmap any exiting mappings from the
operation.  This patch fixes that up.  It also remembers to free the
skb in the event that an error occurs, so we don't leak.  Untested, as
I don't have hardware.  I think its pretty straightforward, but please
review closely.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: Ben Hutchings <bhutchings@solarflare.com>
CC: Jay Cliburn <jcliburn@gmail.com>
CC: Chris Snook <chris.snook@gmail.com>
CC: "David S. Miller" <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>