drivers: usb_dc_mcux: create thread and connect IRQ during init

So far thread was created as part of usb_dc_attach() by k_thread_create().
This means that if following function were executed:

 * usb_enable()
 * usb_disable()
 * usb_enable()

then k_thread_create() was called second time. This results in undefined
behavior.

Fix above issue by moving k_thread_create() invocation to function called
during system initialization.

While at it, move IRQ_CONNECT() and irq_enable() invocations to init as
well.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>