staging: most: usb: don't set URB_ZERO_PACKET flag for synchronous data

This patch avoids setting the URB_ZERO_PACKET transfer flag for synchronous
data. This is needed to prevent the host from sending an empty packet when
data is aligned to an endpoint packet boundary.

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>