wcn36xx: add debug prints for sw_scan start/complete

Add some MAC debug prints for more easily demarcating a software scan
when parsing logs.

Signed-off-by: Benjamin Li <benl@squareup.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211027170306.555535-2-benl@squareup.com