usbnet: ipheth: transmit URBs without trailing padding

The behaviour of the official iOS tethering driver on macOS is to not
transmit any trailing padding at the end of URBs. This is applicable
to both NCM and legacy modes, including older devices.

Adapt the driver to not include trailing padding in TX URBs, matching
the behaviour of the official macOS driver.

Signed-off-by: Foster Snowhill <forst@pen.gy>
Tested-by: Georgi Valkov <gvalkov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>