usbip: vudc: fix: Clear already_seen flag also for ep0

ep_list inside gadget structure doesn't contain ep0.
It is stored separately in ep0 field.

This causes an urb hang if gadget driver decides to
delay setup handling. On host side this is visible as
timeout error when setting configuration.

This bug can be reproduced using for example any gadget
with mass storage function.

Fixes: abdb2957

 ("usbip: vudc: Add vudc_transfer")
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Cc: stable <stable@vger.kernel.org>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>