Use __SPIN_LOCK_UNLOCKED to initialize bad_irq_desc.lock

SPIN_LOCK_UNLOCKED is deprecated as lockdep cannot properly work with
locks initialized with it.

This fix is necessary to compile the linux-rt tree for ARM.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Steven Rostedt <srostedt@redhat.com>