sis900: fix warning in drivers/net/sis900.c

this warning:

  drivers/net/sis900.c: In function ‘sis900_timer’:
  drivers/net/sis900.c:1280: warning: ‘speed’ may be used uninitialized in this function

triggers because GCC does not recognize the (correct) error flow
between sis900_read_mode(), 'speed' and 'duplex'.

Annotate it.

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