media: cec: core: remove length check of Timer Status

The valid_la is used to check the length requirements,
including special cases of Timer Status. If the length is
shorter than 5, that means no Duration Available is returned,
the message will be forced to be invalid.

However, the description of Duration Available in the spec
is that this parameter may be returned when these cases, or
that it can be optionally return when these cases. The key
words in the spec description are flexible choices.

Remove the special length check of Timer Status to fit the
spec which is not compulsory about that.

Signed-off-by: Nini Song <nini.song@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>