wcd9335: fix a incorrect use of kstrndup()

In wcd9335_codec_enable_dec(), 'widget_name' is allocated by kstrndup().
However, according to doc: "Note: Use kmemdup_nul() instead if the size
is known exactly." So we should use kmemdup_nul() here instead of
kstrndup().

Signed-off-by: Gen Zhang <blackgod016574@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>