usb: dwc2: disable power_down on Amlogic devices

Disable power_down by setting the parameter to
DWC2_POWER_DOWN_PARAM_NONE. This fixes a problem on various Amlogic
Meson SoCs where USB devices are only recognized when plugged in before
booting Linux. A hot-plugged USB device was not detected even though the
device got power (my USB thumb drive for example has an LED which lit
up).

A similar fix was implemented for Rockchip SoCs in commit c216765d
("usb: dwc2: disable power_down on rockchip devices"). That commit
suggests that a change in the dwc2 driver is the cause because the
default value for the "hibernate" parameter (which then got renamed to
"power_down" to support other modes) was changed in the v4.17 merge
window with:
commit 6d23ee9c ("Merge tag 'usb-for-v4.17' of git://git.kernel.org/pub/scm/linux/kernel/git/balbi/usb

 into usb-testing").

Cc: <stable@vger.kernel.org> # 4.19
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Suggested-by: Christian Hewitt <christianshewitt@gmail.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>