libertas: fix use-before-check violation

According to Coverity (kudo's to Adrian Bunk), we had one use-before-check
bug in libe libertas driver. This patch fixes this issue.

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