usb: smd: Send zero length packets in modem data calls

The USB protocol requires zero length packets to be sent when the data
transmission ends on a USB packet size boundary. This fix enables sending
zero length packets when required.

Change-Id: Ife4edae6e0ef01d0e06e5a9e0e29cf25fb6e7465
CRs-fixed: 546473
Signed-off-by: Danny Segal <dsegal@codeaurora.org>