b43: Fix possible NULL pointer dereference in DMA code

This fixes a possible NULL pointer dereference in an error path of the
DMA allocation error checking code. This is also necessary for a future
DMA API change that is on its way into the mainline kernel that adds
an additional dev parameter to dma_mapping_error().

This patch moves the whole struct b43_dmaring struct initialization
right before any DMA allocation operation.

Reported-by: Miles Lane <miles.lane@gmail.com>
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>