6lowpan: no need to check return value of debugfs_create functions

When calling debugfs functions, there is no need to ever check the
return value.  The function can work or not, but the code logic should
never do something different based on this.

Because we don't care if debugfs works or not, this trickles back a bit
so we can clean things up by making some functions return void instead
of an error value that is never going to fail.

Cc: Alexander Aring <alex.aring@gmail.com>
Cc: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-bluetooth@vger.kernel.org
Cc: linux-wpan@vger.kernel.org
Cc: netdev@vger.kernel.org
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>