sys-firmware/b43-firmware: mask versions above 5.100.138

sys-firmware/b43-firmware: mask versions above 5.100.138

Any sys-firmware/b43-firmware version above 5.100.138 breaks b43 driver in
various ways (spontaneous disconnections, low throughput, cannot associate
to an AP, etc).
Also, b43 wiki page says to use 5.100.138:
http://linuxwireless.org/en/users/Drivers/b43/#Other_distributions_not_mentioned_above

Let's hard mask 6.30.163.46 (next version after 5.100.138) and above so
users' wireless card don't suddenly break when they update @world.

Bug: https://bugs.gentoo.org/541080
Closes: https://github.com/gentoo/gentoo/pull/3491