usb: device: Fix ZLP write race condition

ZLP packet has to be read and acknowledged by host just like any other
DATA packet. Do not end transfer until the host actually acknowledged
the trailing ZLP. This fixes the race condition between host and Zephyr
application where the next transfer could be lost if host did not issue
IN token (that would read read ZLP) before the application tried to
start new transfer.

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