mac80211_hwsim: Group radios

Currently all radios receive all traffic on the simulated air
if they are tuned to the same channel. This patch introduces
the concept of grouping, which allows to assign a radio to
certain group. Only radios in the same group can 'see' each other.

Each bit in /debug/ieee80211/phy*/hwsim/group
represents one group. By default all radios belong to the same group "1",
e.g. bit 1 is set. Additionally a radio can belong to several groups.

Signed-off-by: Daniel Wagner <wagi@monom.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>