usbnet: avoid integer overflow in start_xmit

transfer_buffer_length is of type u32. It's therefore wrong to assign it
to a signed integer. This patch avoids the overflow.

It's worth noting that entry->length here is a long; perhaps it would be
beneficial at somepoint to change this to be unsigned as well, if
nothing else relies on its signedness for error conditions or the like.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>