forcedeth: Fix WARNINGS that result when DMA mapping is not checked

With 3.8-rc1, the first call of pci_map_single() that is not checked
with a corresponding pci_dma_mapping_error() call results in a warning
with a splat as follows:

WARNING: at lib/dma-debug.c:933 check_unmap+0x480/0x950()
Hardware name: HP Pavilion dv2700 Notebook PC
forcedeth 0000:00:0a.0: DMA-API: device driver failed to check
 map error[device address=0x00000000b176e002] [size=90 bytes] [mapped as single]

Reviewed-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: David S. Miller <davem@davemloft.net>