iwlwifi: mvm: fix merge damage in iwl_mvm_vif_dbgfs_register()

When I rebased Greg's patch, I accidentally left the old if block that
was already there.  Remove it.

Fixes: 154d4899 ("iwlwifi: mvm: properly check debugfs dentry before using it")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>