[efi] Report any USB errors as EFI_USB_ERR_SYSTEM

[efi] Report any USB errors as EFI_USB_ERR_SYSTEM

Some UEFI USB drivers (e.g. the UsbKbDxe driver in EDK2) will react to
a reported EFI_USB_ERR_STALL by attempting to clear the endpoint halt.
This is redundant with iPXE's EFI_USB_IO_PROTOCOL implementation,
since endpoint stalls are cleared automatically by the USB core as
needed.

The UEFI USB driver's attempt to clear the endpoint halt can introduce
an unwanted 5 second delay per endpoint if the USB error was the
result of a device being physically removed, since the control
transfer will always time out.

Fix by reporting all USB errors as EFI_USB_ERR_SYSTEM instead of
EFI_USB_ERR_STALL.

Signed-off-by: Michael Brown <mcb30@ipxe.org>