usb: dwc3: Kconfig: get rid of obsolete mode selection

usb: dwc3: Kconfig: get rid of obsolete mode selection

The mode selection for the DWC3 is kind of obsolete. The driver does not
have to be host only or gadget only. This choice is confusing.
All the remaining callers of dwc3_uboot_init() explicitly set dr_mode
before calling the function, so none rely on a default behavior.

Remove the choice menu and keep only the USB_DWC3_GADGET option. Enable it
by default if USB_GADGET and USB_DWC3 are enabled.
It must be disabled for the evb-rk3328 as it uses DWC2 for the gadget and
DWC3 for the host.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>