usb: cdc-acm: send ZLP for Telit 3G Intel based modems

commit 34aabf918717dd14e05051896aaecd3b16b53d95 upstream.

Telit 3G Intel based modems require zero packet to be sent if
out data size is equal to the endpoint max packet size.

Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>