brcmfmac: correct .disconnect() callback while connecting

When the driver has sent a join iovar to the firmware it waits
for the events to report result of the connection. However, the
wpa_supplicant will request a .disconnect() after a timeout. So
upon calling .disconnect() the interface state may still be
CONNECTING. Clear the CONNECTING bit as well.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>