mac80211, hostapd: always explicitly set beacon interval

One of the latest mac80211 updates added sanity checks, requiring the
beacon intervals of all VIFs of the same radio to match. This often broke
AP+11s setups, as these modes use different default intervals, at least in
some configurations (observed on ath9k).

Instead of relying on driver or hostapd defaults, change the scripts to
always explicitly set the beacon interval, defaulting to 100. This also
applies the beacon interval to 11s interfaces, which had been forgotten
before. VIF-specific beacon_int setting is removed from hostapd.sh.

Fixes FS#619.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>