iwlwifi: fix flow handler debug code

iwl_dbgfs_fh_reg_read() can cause crashes and/or
BUG_ON in slub because the ifdefs are wrong, the
code in iwl_dump_fh() should use DEBUGFS, not
DEBUG to protect the buffer writing code.

Also, while at it, clean up the arguments to the
function, some code and make it generally safer.

Cc: stable@vger.kernel.org
Reported-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/pcie/internal.h

@@ -350,7 +350,7 @@ int iwl_queue_space(const struct iwl_queue *q);
 /*****************************************************
 * Error handling
 ******************************************************/
-int iwl_dump_fh(struct iwl_trans *trans, char **buf, bool display);
+int iwl_dump_fh(struct iwl_trans *trans, char **buf);
 void iwl_dump_csr(struct iwl_trans *trans);
 
 /*****************************************************