libertas: Remove carrier signaling from the scan code

There is no reason to signal a carrier off when doing a 802.11 scan.

Cc: Holger Schurig <holgerschurig@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>