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>