iwlwifi: move iwl_dump_csr to agn

The iwl_dump_csr function is only used
within the agn module, so it can be
moved there instead of being exported
by the core.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>