iwlwifi: mvm: move interface-specific debugfs to a new file

The debugfs file is getting pretty large and mixed up between
code for the hardware and code for each interface (the two
aren't even clearly separated in the code). Make it easier to
handle by splitting the per-interface code into a separate
file called debugfs-vif.c.

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