iwlwifi: mvm: properly check debugfs dentry before using it

iwlwifi: mvm: properly check debugfs dentry before using it

debugfs can now report an error code if something went wrong instead of
just NULL.  So if the return value is to be used as a "real" dentry, it
needs to be checked if it is an error before dereferencing it.

This is now happening because of ff9fb72bc077 ("debugfs: return error
values, not NULL").  If multiple iwlwifi devices are in the system, this
can cause problems when the driver attempts to create the main debugfs
directory again.  Later on in the code we fail horribly by trying to
dereference a pointer that is an error value.

Reported-by: Laura Abbott <[email protected]>
Reported-by: Gabriel Ramirez <[email protected]>
Cc: Johannes Berg <[email protected]>
Cc: Emmanuel Grumbach <[email protected]>
Cc: Luca Coelho <[email protected]>
Cc: Intel Linux Wireless <[email protected]>
Cc: Kalle Valo <[email protected]>
Cc: stable <[email protected]> # 5.0
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Luca Coelho <[email protected]>