staging: brcm80211: bugfix for fifo problem on 64 bits platforms.

Message 'receive fifo overflow' appeared in the log. Root cause was an
invalid physical address being programmed into the DMA engine. This was
caused by an invalid pointer cast in the dma code. Issue was observed on
a Sparc (Sun Fire V120) machine but could theoretically also pop up
on other architectures.

Driver was tested to scan and ping on aforementioned machine.

Reviewed-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>