iwlwifi: clean up transport debugfs handling

Transport code currently calls itself through the transport ops,
which is quite pointless. Clean up all of this. While at it,
remove the unnecessary dir argument and the redundant IDI code.

In slave transports, call both the common slave debugfs and the
transport's own. SDIO has no files, so remove it all there.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>