media: cec-adap.c: don't unconfigure if already unconfigured

The __cec_s_log_addrs() function can configure or unconfigure the
adapter. The ioctl handler in cec-api.c will prevent it from being
called to configure the adapter if it was already configured (or in
the process of configuring). But it can still be called to unconfigure
an already unconfigured adapter, and it didn't check for that.

This can cause cec_activate_cnt_dec() to be called too often, causing
a WARN_ON.

Instead first check if adap->log_addrs.num_log_addrs == 0 and return
since in that case the adapter is already unconfigured.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>