rt2x00: 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 need to save the individual debugfs files and
directories, remove the local storage of them and just remove the entire
debugfs directory in a single call, making things a lot simpler.

Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Helmut Schaa <helmut.schaa@googlemail.com>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>