btrfs: Suppress the message about missing filesystem

This message comes up a lot when scanning filesystems. It suggests to the
user that there is some sort of error, but in fact there is no reason to
expect that a particular partition has a btrfs filesystem. Other
filesystems don't print this error.

Turn it into a debug message.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Qu Wenruo <wqu@suse.com>