mwifiex: scan command preparation failure handling

When scan request is received, scan commands are prepared and
queued into scan pending queue. There is a corner case when
command nodes are full. So we stop queueing further scan
commands and return an error. This patch makes sure that
currently queued commands in scan pending queue are also freed
in this case.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>