iwlwifi: mvm: add debugfs hook to send ECHO_CMD to the firmware

ECHO_CMD is a simple command that can be sent to the
firmware just to check that it is alive.
This command is useful for debug purpose, provide a debugfs
hook to send it.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>