p54usb: fix packet loss with first generation devices

Artur Skawina confirmed that the first generation devices needs the same
URB_ZERO_PACKET flag, in oder to finish the pending transfer properly.
The second generation has been successfully fixed by
"p54usb: fix random traffic stalls (LM87)" (43af18f06d5)

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>