net/ps3_gelic_net: Use dma_mapping_error

The current Gelic Etherenet driver was checking the return value of its
dma_map_single call, and not using the dma_mapping_error() routine.

Fixes runtime problems like these:

  DMA-API: ps3_gelic_driver sb_05: device driver failed to check map error
  WARNING: CPU: 0 PID: 0 at kernel/dma/debug.c:1027 .check_unmap+0x888/0x8dc

Fixes: 02c1889166b4 (ps3: gigabit ethernet driver for PS3, take3)
Signed-off-by: Geoff Levand <geoff@infradead.org>