mwifiex: return -EBUSY if specific scan request cannot be honored

mwifiex: return -EBUSY if specific scan request cannot be honored

Previous patch "mwifiex: return -EBUSY if scan request cannot.."
corrected regular scan request only. There is another case for
specific scan that needs the same handling.

Also, removed !req_ssid check as it has already been validated
by caller.

Signed-off-by: Bing Zhao <[email protected]>
Signed-off-by: Paul Stewart <[email protected]>
Reviewed-by: Ryan Cairns <[email protected]>
Signed-off-by: Amitkumar Karwar <[email protected]>
Signed-off-by: John W. Linville <[email protected]>