libertas: use spin_is_locked() instead of spin_trylock() in lbs_interrupt()

We get scary warnings on UP if we use spin_trylock() and find, as we
hoped, that the lock in question is already locked.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>