wcn36xx: Fix software-driven scan

For software-driven scan, rely on mac80211 software scan instead
of internal driver implementation. The internal implementation
cause connection trouble since it keep the antenna busy during
the entire scan duration, moreover it's only a passive scanning
(no probe request). Therefore, let mac80211 manages sw scan.

Note: we fallback to software scan if firmware does not report
scan offload support or if we need to scan the 5Ghz band (currently
not supported by the offload scan...).

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1598288035-19790-1-git-send-email-loic.poulain@linaro.org