adb: fix spurious usb write failure on Windows.

We were accidentally returning 0 instead of the number of bytes written
whenever we wrote a USB packet that had a size that was a multiple of
the USB packet size, which resulted in the device getting kicked.

Bug: http://b/113070258
Test: python test_device.py
Change-Id: Ib3d6415545e90e1f4730afc8ad8713d10bb1534a