net: usb: Initialize retval with zero as it might be used uninitialized

If at the start of usbnet_start_xmit function sk_buff is null then
there is a chance that retval could be used uninitialized. To avoid
such scenario initialize retval with zero.

Change-Id: I2d5b68075338f7d19b6bc76a4ce7cdbe422daadb
Signed-off-by: Rohith Kollalsi <rkollals@codeaurora.org>