mac80211: Do not build b43legacy on BRCM47xx mips74 subtarget

mac80211: Do not build b43legacy on BRCM47xx mips74 subtarget

b43legacy needs ssb support and we do not compile the mips74 subtarget
of the brcm47xx target with SSB support. This causes a build failure in
the mac80211 package and only some of the kernel modules are being
created.

I am not aware of any device with a BRCM47xx mips74 CPU which uses a
b43legacy compatible device.

Fixes: FS#2334
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit e05310b9b80f944c718374b449b6fe48d06d412d)