USB: dummy-hcd: Add missing annotation for set_link_state()

Sparse reports a warning at set_link_state()

warning: context imbalance in set_link_state() - unexpected unlock

The root cause is the missing annotation at set_link_state()
Add the missing __must_hold(&dum->lock)
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

drivers/usb/gadget/udc/dummy_hcd.c

@@ -427,6 +427,7 @@ static void set_link_state_by_speed(struct dummy_hcd *dum_hcd)
 
 /* caller must hold lock */
 static void set_link_state(struct dummy_hcd *dum_hcd)
+	__must_hold(&dum->lock)
 {
 	struct dummy *dum = dum_hcd->dum;
 	unsigned int power_bit;