drivers/net: Add missing unlock

Unlock the lock before leaving the function.

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

)

// <smpl>
@r exists@
expression E1;
identifier f;
@@

f (...) { <+...
* spin_lock_irqsave (E1,...);
... when != E1
* return ...;
...+> }
// </smpl>

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