Reject new SCAN commands if there is a pending request

Reject new SCAN commands if there is a pending request

FAIL-BUSY was already returned for the case where a scan had been
started, but with the radio work design, it would have been possible to
schedule multiple scan requests if a non-scan radio work was in
progress. Multiple back-to-back scans are not usually very helpful, so
reject this type of cases where the SCAN command would be used to build
such a sequence.

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