iwlagn: wait for asynchronous firmware loading

When we kick off a firmware loading process,
and then unbind from the pci device right
away, we get into trouble. Avoid that by
waiting for the firmware loading to finish
(whether successfully or not) before the
unbind in iwl_pci_remove.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>