mwifiex: fix scan_block flag handling

scan_block flag is used to block scan operation when 4 way handshake
is in progress. Sometimes it doesn't get cleared due to incomplete
association. An example is assoc request/response is done, but add key
operation get canceled in some corner cases. As a result, further
association/scan operations are blocked.

This patch fixes the problem by clearing scan_block flag.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>