Disable kgdboc failed by echo space to /sys/module/kgdboc/parameters/kgdboc

Echo "" to /sys/module/kgdboc/parameters/kgdboc will fail with "No such
device” error.

This is caused by function "configure_kgdboc" who init err to ENODEV
when the config is empty (legal input) the code go out with ENODEV
returned.

Fixes: 2dd45316 ("kgdboc: Fix restrict error")
Signed-off-by: Wentao Wang <witallwang@gmail.com>
Cc: stable <stable@vger.kernel.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>