tg3: Eliminate tg3_write_sig_post_reset() prototype

This patch moves the implementation of tg3_write_sig_post_reset()
earlier 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>