rockchip: use standard dr_mode parsing function

Instead of duplicating the string values here, use usb_get_dr_mode() to
handle the property lookup and converting the values to an enum.

This is implemented with a switch in preparation for the next patch
which adds extra handling for peripheral mode.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>