usb: common: add helper to get role-switch-default-mode

Add helper to get "role-switch-default-mode", and convert it
to the corresponding enum usb_dr_mode

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1626340078-29111-6-git-send-email-chunfeng.yun@mediatek.com


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