iwlwifi: Check HW ready before prepare card.

Hardware may be ready for us to manage it without us trying to prepare
it first. Check if this is the case.

Signed-off-by: Mohamed Abbas <mohamed.abbas@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>