iwlagn: implement synchronous firmware load

The current firmware loading mechanism in
iwlwifi is very hard to follow, and thus
hard to maintain. To make it easier, make
the firmware loading synchronous.

For now, as a side effect, this removes a
number of retry possibilities we had. It
isn't typical for this to fail, but if it
does happen we restart from scratch which
this also makes easier to do should it be
necessary.

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