Fix 20/40 MHz co-ex report processing with obss_interval=0

Fix 20/40 MHz co-ex report processing with obss_interval=0

If OBSS scan interval is not set, the AP must not schedule a timeout to
restore 40 MHz operation immediately after having moved to a 20 MHz
channel based on an unsolicited co-ex report. Fix this by scheduling the
timeout only if obss_interval is non-zero.

Since we do not currently support AP doing OBSS scans after the initial
BSS setup, this means practically that 40-to-20 MHz transition is
allowed, but 20-to-40 MHz is not with obss_interval=0. The latter gets
enabled if obss_interval is set to a non-zero value so that associated
STAs can take care of OBSS scanning.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>