usb: dwc2: use well defined macros for power_down

Use the well defined macros such as DWC2_POWER_DOWN_PARAM_NONE,
DWC2_POWER_DOWN_PARAM_PARTIAL and DWC2_POWER_DOWN_PARAM_HIBERNATION
to make code more readable.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Link: https://lore.kernel.org/r/20200616162617.38365cc8@xhacker.debian


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