libertas: Check return status of command functions

Return status of lbs_prepare_and_send_command and lbs_cmd_with_response
functions is not checked at some places. Those checks are added.

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>