libertas: fix scheduling while atomic bug in CMD_MAC_CONTROL

The old code incorrectly used lbs_cmd_with_response() and now uses
lbs_cmd_async().

While there I noticed that there is no real useful return values for
asynchronous command functions, so I made the function "void".

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>