cmd: mbr: Force DOS driver to be used for verify

Forces the DOS partition type driver to be used when verifying the MBR.
This is particularly useful when using a hybrid MBR & GPT layout as
otherwise MBR verification would mostly likely fail since the GPT
partitions will be returned, even if the MBR is actually valid.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>