hostapd: explicitly set beacon interval for wpa_supplicant

hostapd: explicitly set beacon interval for wpa_supplicant

The beacon_int is currently set explicitly for hostapd and when LEDE uses
iw to join and IBSS/mesh. But it was not done when wpa_supplicant was used
to join an encrypted IBSS or mesh.

This configuration is required when an AP interface is configured together
with an mesh interface. The beacon_int= line must therefore be re-added to
the wpa_supplicant config. The value is retrieved from the the global
variable.

Fixes: 1a16cb9c67f0 ("mac80211, hostapd: always explicitly set beacon interval")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [rebase]
(cherry picked from commit 772afef61dc68e2470f4da130fac862ccf2fb105)