b44: fix workarround for wap54g10

The code for the b44_wap54g10_workaround was never included, because
the config option was wrong. The nvram_get function was never in
mainline kernel, only in external OpenWrt patches.

The code should be compiled in when CONFIG_BCM47XX is selected and not
when CONFIG_SSB_DRIVER_MIPS is selected, because nvram_getenv is only
available on bcm47xx platforms and now in the mainline kernel code.
Using an include is better than a second function declaration, to fix
this when the function signature changes.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>