rtl818x: check for pci_map_single() success when initializing RX ring

During initialization a number of RX skbs are allocated and mapped
for DMA.
Currently if pci_map_single() fails, it will result in passing to the
HW a wrong DMA address (to write to!).

This patch adds check for this condition and eventually causes the
driver not to initialize, avoiding at least dangerous DMAs.

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