spidernet: load firmware when open

This patch moves calling init_firmware() from spider_net_probe() to
spider_net_open() so as to use the driver by built-in.

Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>