iwlagn: Rename iwlcore prefix

There are number of functions with "iwlcore_" prefix which not feels right,
rename those to "iwl_".

No functional changes by making the renames.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>