phy-sun4i-usb: Refactor forced session ending

The phy-sun4i-usb code supports forced ending a session on systems
which lack Vbus detection, to allow switching between host and peripheral
mode on such systems.

Role switching via the musb driver "mode" sysfs attribute requires force
ending the session too. This commit refactors the code to allow other
parts of the phy-sun4i-usb code to request a forced session end.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>