ipw2100: fix rebooting hang with driver loaded

Add PCI .shutdown method so that we can disable the device during
shutdown or reboot. Without this, the reboot doesn't work well on
some platforms.

This fixes http://bugzilla.intellinuxwireless.org/show_bug.cgi?id=2124



Tested-by: pablo <pablolm2005@gmail.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>