drivers: usb_dc_native_posix: do callback for ZLPs

A Zero Length Packet can be used by higher layer stack to discover when
an endpoint is being processed by the host. An example of this was
introduced as part of 0127d000 ("usb: device: cdc_acm: Use ZLP to
detect initial host read") in the CDC ACM class.

Not invoking the callback for ZLPs results in the higher layer stack not
being informed when the packet is consumed. This manifests as a CDC ACM
USB-IP device that cannot transmit to the host while being able to
receive from the host.

Signed-off-by: Abe Kohandel <abe.kohandel@gmail.com>