iwlwifi: make sure device is reset when unloading driver

Add unconditional call to apm_ops.stop() to reset device to low power state
when unloading driver.  Some paths have existed to unload driver *without*
resetting device, therefore some errors have persisted through multiple
load/unload cycles, until the whole platform gets rebooted; this is an
attempt to remedy some of those situations.  Sorry, I can't seem to find
a bughost.org bug that specifically has these symptoms, but I had it happen
recently here.

Note that this will *not* fix situations in which the PCI express bus has
crashed (evidenced by register reads showing "0xffffffff"), e.g. bughost.org
1855 and 2096; device is unreachable from driver in those cases.

Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>