usb: musb: dsps: polling ID pin status only in otg mode

Only start the otg_timer in dual role mode; otherwise in peripheral mode
when musb is disconnected from the host port, otg_timer starts and
continuously toggles the session, which causes VBUS pulse.

Signed-off-by: Bin Liu <b-liu@ti.com>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>