brcm47xx: fix kmod-usb-brcm47xx build on legacy and mips74k subtargets

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41052 3c298f89-4303-0410-b956-a3cf2f4a3e73
brcm47xx: merge FEATURES lines in the legacy subtarget

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41053 3c298f89-4303-0410-b956-a3cf2f4a3e73