drm/amd/pm: Handle non-terminated overdrive commands.

commit 08e9ebc7 upstream.

The incoming strings might not be terminated by a newline
or a 0.

(found while testing a program that just wrote the string
 itself, causing a crash)

Cc: stable@vger.kernel.org
Fixes: e3933f26

 ("drm/amd/pp: Add edit/commit/show OD clock/voltage support in sysfs")
Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>