drivers: usb_dc_nrfx: Abort write on ep_ctx_reset

Change adds abort of ongoing write operation in ep_ctx_reset. This is
required to keep the state of Zephyr driver consistent with state of
nrfx driver. This fixes a bug where nrfx_usbd was stuck in busy state.

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>