libertas: Increase priority of 'unknown command' warnings

Using the deprecated lbs_prepare_and_send_command() function for a
command which it doesn't understand is an error; complain loudly about
it even when we're not debugging.

The mesh stats bug, where we converted MESH_ACCESS to a direct command
but accidentally missed one user which was still trying to do it through
lbs_prepare_and_send_command(), would have been caught a lot quicker if
we'd done this sooner. Such bugs aren't entirely unlikely in future too,
as we convert more code to stop using this function.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>