niu: fix another warning in drivers/net/niu.c

this warning:

  drivers/net/niu.c: In function ‘esr_reset’:
  drivers/net/niu.c:741: warning: ‘reset’ may be used uninitialized in this function

triggers because GCC does not recognize the (correct) error flow
between:

 - esr_read_reset() and 'reset'

Annotate it.
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/niu.c

@@ -738,7 +738,7 @@ static int esr_write_glue0(struct niu *np, unsigned long chan, u32 val)
 
 static int esr_reset(struct niu *np)
 {
-	u32 reset;
+	u32 uninitialized_var(reset);
 	int err;
 
 	err = mdio_write(np, np->port, NIU_ESR_DEV_ADDR,