libertas: fix spinlock recursion bug

This fixes a bug detected by CONFIG_DEBUG_SPINLOCK:

if_cs_get_int_status() is only called from lbs_thread(), via
priv->hw_get_int_status. However, lbs_thread() has already taken the
priv->driver_lock. So it's a fault to take the same lock again here.

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>