rtl8180: Add error check for pci_map_single return value in TX path

Orignal code will not detect a DMA mapping failure, causing the HW
to attempt a DMA from an invalid address.

This patch add the error check and eventually simply drops the TX
packet if we can't map it for DMA.

Signed-off-by: andrea merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>