usb: dwc3: replace hard-coded constant in DWC3_GCTL_SCALEDOWN(3)

Define DWC3_GCTL_SCALEDOWN_MASK and use it in place of
DWC3_GCTL_SCALEDOWN(3).

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>