drivers: usb: mcux: mark endpoint unoccupied on disable

In usb_dc_ep_disable() mark an endpoint as unoccupied in addition to
being disabled. This allows the endpoint to get enabled properly
with a subsequent call to usb_dc_ep_enable()

See: zephyrproject-rtos/zephyr#51685

Signed-off-by: Milind Paranjpe <mparanjpe@yahoo.com>