wic: do not use PARTLABEL for msdos partition tables

When using "msdos" partition tables and "--label" but not "--use-uuid"
one can generate images which will not find their root, because
PARTLABEL does not work for "msdos".

Fix that by simply not going the PARTLABEL path in case of "msdos".

Fixes: 2fb247c5 ("wic: support rootdev identified by partition label")
Signed-off-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>