iwlwifi: move rate registration to module load

Having rate registration during module load enables the use of
error checking as well as reliable registration/unregistration
pairing. Previously this was not possible as rate registration
was done during _probe where _probe could be run for more than
one device on the system.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>