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 <bzhao@marvell.com>
Signed-off-by: Paul Stewart <pstew@chromium.org>
Reviewed-by: Ryan Cairns <rtc@chromium.org>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>