usb: roles: add helper usb_role_string()

Introduces usb_role_string() function, which returns a
human-readable name of provided usb role, it's useful to
make the log readable.

Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1621932786-9335-1-git-send-email-chunfeng.yun@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>