lan78xx: Fix memory leaks

In lan78xx_probe(), a new urb is allocated through usb_alloc_urb() and
saved to 'dev->urb_intr'. However, in the following execution, if an error
occurs, 'dev->urb_intr' is not deallocated, leading to memory leaks. To fix
this issue, invoke usb_free_urb() to free the allocated urb before
returning from the function.

Signed-off-by: Wenwen Wang <wenwen@cs.uga.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>