usb: musb: sunxi: Simplify dr_mode handling

phy-sun4i-usb now has proper dr_mode handling, it always registers an
extcon, and sends a notify with the mode (even when in peripheral- /
host-only mode) at least once.

So we can simply the sunxi musb glue by always registering its extcon
notifier and relying on sunxi_musb_work() to enable vbus when in
host-only mode.

This also enables host- and peripheral-only mode with vbus monitoring.

Tested-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>