cfg80211: support virtual interfaces with different beacon intervals

This commit provides a mechanism for the host drivers to advertise the
support for different beacon intervals among the respective interface
combinations in a group, through NL80211_IFACE_COMB_BI_MIN_GCD (u32).

This value will be compared against GCD of all beaconing interfaces of
matching combinations.

If the driver doesn't advertise this value, the old behaviour where
all beacon intervals must be identical is retained.

If it is specified, then any beacon interval for an interface in the
interface combination as well as the GCD of all active beacon intervals
in the combination must be greater or equal to this value.

Signed-off-by: Purushottam Kushwaha <pkushwah@qti.qualcomm.com>
[change commit message, some variable names, small other things]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>