drivers: usb: nordic: Fix IN transfer fragmentation

Currently, when usb_dc_ep_write is called, data is copied to the
internal buffer, hence the requirement for fragmentation, regardless
of low-level nrfx_usbd driver. This commit forces the fragmentation
and prevents potential internal buffer overflow. Adittional
fragmentation flag was added to prevent triggering status stage
(it is handled by hardware completely on nRF chips).

Fixes #12339

Signed-off-by: Paweł Zadrożniak <pawel.zadrozniak@nordicsemi.no>