libertas: fix WPA adhoc network creation

Oddly enough, the firmware's JOIN/START commands don't appear to have
any facility for setting custom IEs, thus the started adhoc network
doesn't advertise its WPA capability in the beacon.  Whee!

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>