iwlagn: use iwl_eeprom_calib_hdr structure

For retrieve calibration hdr related information, instead of using structure in
one place and #define in other place, unify the method to use data structure.

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