iwlwifi: add FW_INFO debug level

Add FW_INFO debug level. This level is enabled if INFO or FW debug
levels are set.
Also, set fw request and callback prints under this debug level.

Signed-off-by: Shahar S Matityahu <shahar.s.matityahu@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>