staging: rtl8192e: Fix failure to check pci_map_single()

Beginning with kernel 3.8, the DMA mapping routines issue a warning
for the first call to pci_map_single() that is not checked with a
pci_dma_mapping_error() call.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>