usb: roles: Leave the private driver data pointer to the drivers

Adding usb_role_switch_get/set_drvdata() functions that the
switch drivers can use for setting and getting private data
pointer that is associated with the switch.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20200302135353.56659-5-heikki.krogerus@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>