usb: dfu: Schedule timer handler from USB workqueue

Use k_work_delayable instead of k_timer in order to execute timeout in
USB workqueue context instead of ISR context. This fixes Will-Detach on
targets where usb_dc_detach() uses functions not allowed in ISR context,
e.g. nrfx usb_dc_detach() acquires mutex.

Fixes: c27d48c8 ("usb: dfu: Support DFU with WinUSB on Windows")

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>