usb: dwc3: gadget: Remove invalid low-speed setting

None of the DWC_usb3x IPs (and all their versions) supports low-speed
setting in device mode. In the early days, our "Early Adopter Edition"
DWC_usb3 databook shows that the controller may be configured to operate
in low-speed, but it was revised on release. Let's remove this invalid
speed setting to avoid any confusion.

Signed-off-by: Thinh Nguyen <[e-mail]>
Link: https://lore.kernel.org/r/258b[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/dwc3/core.c

···
 
 	/* Check the maximum_speed parameter */
 	switch (dwc->maximum_speed) {
-	case USB_SPEED_LOW:
 	case USB_SPEED_FULL:
 	case USB_SPEED_HIGH:
 		break;

drivers/usb/dwc3/core.h

···
 #define DWC3_DCFG_SUPERSPEED	(4 << 0)
 #define DWC3_DCFG_HIGHSPEED	(0 << 0)
 #define DWC3_DCFG_FULLSPEED	BIT(0)
-#define DWC3_DCFG_LOWSPEED	(2 << 0)
 
 #define DWC3_DCFG_NUMP_SHIFT	17
 #define DWC3_DCFG_NUMP(n)	(((n) >> DWC3_DCFG_NUMP_SHIFT) & 0x1f)
···
 #define DWC3_DSTS_SUPERSPEED		(4 << 0)
 #define DWC3_DSTS_HIGHSPEED		(0 << 0)
 #define DWC3_DSTS_FULLSPEED		BIT(0)
-#define DWC3_DSTS_LOWSPEED		(2 << 0)
 
 /* Device Generic Command Register */
 #define DWC3_DGCMD_SET_LMP		0x01

drivers/usb/dwc3/gadget.c

···
 		reg |= DWC3_DCFG_SUPERSPEED;
 	} else {
 		switch (speed) {
-		case USB_SPEED_LOW:
-			reg |= DWC3_DCFG_LOWSPEED;
-			break;
 		case USB_SPEED_FULL:
 			reg |= DWC3_DCFG_FULLSPEED;
 			break;
···
 		dwc3_gadget_ep0_desc.wMaxPacketSize = cpu_to_le16(64);
 		dwc->gadget->ep0->maxpacket = 64;
 		dwc->gadget->speed = USB_SPEED_FULL;
-		break;
-	case DWC3_DSTS_LOWSPEED:
-		dwc3_gadget_ep0_desc.wMaxPacketSize = cpu_to_le16(8);
-		dwc->gadget->ep0->maxpacket = 8;
-		dwc->gadget->speed = USB_SPEED_LOW;
 		break;
 	}