iwlwifi: Add comments about CSR registers

Also regroup CSR_EEPROM and CSR_OTP bit field definitions.

Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>