powerpc/imc-pmu: Use the correct spinlock initializer.

The macro __SPIN_LOCK_INITIALIZER() is implementation specific. Users
that desire to initialize a spinlock in a struct must use
__SPIN_LOCK_UNLOCKED().

Use __SPIN_LOCK_UNLOCKED() for the spinlock_t in imc_global_refc.

Fixes: 76d588dd ("powerpc/imc-pmu: Fix use of mutex in IRQs disabled section")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230309134831.Nz12nqsU@linutronix.de