USB: serial: option: add ZLP support for 0x1bc7/0x9010

Telit FN980 flashing device 0x1bc7/0x9010 requires zero packet
to be sent if out data size is is equal to the endpoint max size.

Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
[ johan: switch operands in conditional ]
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>