PS3: fix the bug that 'ifconfig down' would hang

Fix the bug that 'ifconfig eth0 down' would hang up, reported by Stefan
Assmann <sassmann@suse.de>.
As we removed netif_poll_enable() from dev->open(), we should not use
netif_poll_disable() in dev->stop().

Signed-off-by: Masakazu Mokuno <mokuno@sm.sony.co.jp>
CC: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>