carl9170: interrupt urbs must not set URB_ZERO_PACKET

This patch fixes a bug in the driver which was
exposed by CONFIG_USB_DEBUG:
	"usb 1-1.6.3: BOGUS urb flags, 40 --> 0"

The transfer flag "URB_ZERO_PACKET" is only valid
for bulk urbs.

Reported-by: André Erdmann
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>