staging: brcm80211: restrict MIPS dma bug workaround to BCM47XX

The inline function dma_spin_for_len() was defined for MIPS platforms
but the problem only occurs with dma of the PCI core in bcm47xx chips.
This patch restricts the function further to BCM47XX platforms only.

Tested on BCM63281.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Henry Ptasinski <henryp@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Tested-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>