3c59x: fix missing dma_mapping_error check and bad ring refill logic

A few spots in 3c59x missed calls to dma_mapping_error checks, casuing
WARN_ONS to trigger.  Clean those up.  While we're at it, refactor the
refill code a bit so that if skb allocation or dma mapping fails, we
recycle the existing buffer.  This prevents holes in the rx ring, and
makes for much simpler logic

Note: This is compile only tested.  Ted, if you could run this and
confirm that it continues to work properly, I would appreciate it, as I
currently don't have access to this hardware

Signed-off-by: Neil Horman <nhorman@redhat.com>
CC: Steffen Klassert <klassert@mathematik.tu-chemnitz.de>
CC: "David S. Miller" <davem@davemloft.net>
Reported-by:  <tedheadster@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>