libertas: don't spin_unlock_irq() twice

priv->driver_lock has already been unlocked some lines above. This patch
fixes the sparse warning:

drivers/net/wireless/libertas/main.c:792:6: warning: context problem in 'lbs_thread': '_spin_unlock_irq' expected different context
drivers/net/wireless/libertas/main.c:792:6:    context 'lock': wanted >= 1, got 0

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