drivers/net/usb: Correct code taking the size of a pointer

sizeof(dev->dev_addr) is the size of a pointer.  A few lines above, the
size of this field is obtained using netdev->addr_len for a call to memcpy,
so do the same here.

A simplified version of the semantic patch that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@@
expression *x;
expression f;
type T;
@@

*f(...,(T)x,...)
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>