BB: brcm47xx: fix brcm47xx.legacy build

we should check for ssb here.

backport of r42164.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>


git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42166 3c298f89-4303-0410-b956-a3cf2f4a3e73