powerpc/eeh: atomic_dec_if_positive() to update passthru count

No need to have two atomic opertions (update and fetch/check) when
decreasing PE's number of passed devices as one atomic operation
is enough.

Signed-off-by: Gavin Shan <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>

arch/powerpc/kernel/eeh.c

···
 		goto out;
 
 	/* Decrease PE's pass through count */
-	atomic_dec(&edev->pe->pass_dev_cnt);
-	WARN_ON(atomic_read(&edev->pe->pass_dev_cnt) < 0);
+	WARN_ON(atomic_dec_if_positive(&edev->pe->pass_dev_cnt) < 0);
 	eeh_pe_change_owner(edev->pe);
 out:
 	mutex_unlock(&eeh_dev_mutex);