iwlagn: avoid hangs when restarting device

If a device error happens while the uCode is
being loaded or initialised, we will attempt
to restart the device (which will likely fail
again, but that's not the issue here). During
this new restart, we turn off the device, but
as the uCode failed to initialise it already
is turned off. As a consequence, grabbing NIC
access will fail and cause excessive messages
and hangs.

To fix this issue, introduce a new status bit
and only attempt to reprogram the device when
it isn't already disabled.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>