usb: dwc3: gadget: clear SUSPHY bit before ep cmds

Synopsys Databook 2.60a has a note that if we're
sending an endpoint command we _must_ make sure that
DWC3_GUSB2PHY(n).SUSPHY bit is cleared.

This patch implements that particular detail.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>