drivers: usb_dc_sam_usbhs: default to full-speed

Use DT_INST_ENUM_IDX_OR and always default to full-speed
if CONFIG_USB_DC_HAS_HS_SUPPORT is not set or maximum-speed
property is not defined.
Remove low-speed setting since device stack does not support it.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>