usb: phy: mv-otg: use to_delayed_work instead of cast

Directly casting a work_struct pointer to a delayed_work is risky if the
work member of struct delayed_work is ever moved from being the first
member.

Instead, use the inline function to_delayed_work(), which does the same
cast in a safer way (using container_of).

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Signed-off-by: Felipe Balbi <balbi@ti.com>