skge: check for PCI dma mapping errors

Driver should check for mapping errors.
Machines with limited DMA maps may return an error when a PCI map is
requested (not an issue on standard x86).

Also use upper/lower 32 bits macros for clarity.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>