libertas: Use lbs_is_cmd_allowed() check in command handling routines.

lbs_is_cmd_allowed() check is added in __lbs_cmd_async() and
lbs_prepare_and_send_command(). The check is removed from other places.

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