P2P: Ignore association events when GO is not active

P2P: Ignore association events when GO is not active

Address an issue observed in the following scenario:
- DUT operating in STA+AP SCC mode on 5200 MHz, with P2P GO initially
  on 5745 MHz.
- Driver PCL restrictions forced GO to switch to 5200 MHz.
- hostapd attempted to revert to 5745 MHz, as 5200 MHz was listed in the
  avoid frequency range reported by the driver.
- Driver again enforced SCC on 5200 MHz.
- Channel switch triggered from hostapd and driver again and again.
- Leftover association events triggered crashes during channel switch
  failure and group restart.

Fix: Add an early check in hostapd_notif_assoc() to ignore association
events when the P2P GO instance is not started or has been disabled.

Signed-off-by: Huang Chenming <chenhuan@qti.qualcomm.com>