tg3: Eliminate tg3_halt_cpu() prototype

This patch moves the implementatino of tg3_halt_cpu() earlier in the
file to eliminate its prototype.
Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>