fs: avoid superfluous messages

Output like the following is quite irritating:

    => bootefi hello
    Scanning disk mmc2.blk...
    No valid Btrfs found
    Bad magic number for SquashFS image.
    ** Unrecognized filesystem type **
    Scanning disk mmc1.blk...
    No valid Btrfs found
    Bad magic number for SquashFS image.
    ** Unrecognized filesystem type **
    Scanning disk mmc0.blk...
    No valid Btrfs found
    Bad magic number for SquashFS image.
    ** Unrecognized filesystem type **

Albeit a whole disk may be formatted with a filesystem in most cases
a partition table is used and the whole disk (partition number 0) doesn't
contain a filesytem. Some partitions may only contain a blob. Not seeing a
filesytem on the whole disk or on any partition is only worth a debug
message.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>