linux_usbfs: Don't mark init_count as volatile

linux_usbfs: Don't mark init_count as volatile

There is no reason to mark init_count as volatile, it is protected by the
linux_hotplug_startstop_lock. Removing the volatile marking fixes the
following Coverity warning:

*** CID 62574:  Side effect in assertion  (ASSERT_SIDE_EFFECT)
/libusb/os/linux_usbfs.c: 460 in op_exit()
454     	return r;
455     }
456
457     static void op_exit(void)
458     {
459     	usbi_mutex_static_lock(&linux_hotplug_startstop_lock);
>>>     CID 62574:  Side effect in assertion  (ASSERT_SIDE_EFFECT)
>>>     Argument "init_count" of assert() has a side effect because the variable is volatile.  The containing function might work differently in a non-debug build.
460     	assert(init_count != 0);
461     	if (!--init_count) {
462     		/* tear down event handler */
463     		(void)linux_stop_event_monitor();
464     	}
465     	usbi_mutex_static_unlock(&linux_hotplug_startstop_lock);

Signed-off-by: Hans de Goede <hdegoede@redhat.com>