niu: fix warnings in drivers/net/niu.c

these warnings:

  drivers/net/niu.c: In function ‘serdes_init_niu_1g_serdes’:
  drivers/net/niu.c:451: warning: ‘sig’ may be used uninitialized in this function
  drivers/net/niu.c: In function ‘serdes_init_niu_10g_serdes’:
  drivers/net/niu.c:550: warning: ‘sig’ may be used uninitialized in this function

triggers because GCC does not recognize that the max_retry loop
always initializes 'sig', due to max_retry != 0.

Annotate them.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>