disk: dos: Infer MBR partition sector size from underlying drive sector size

Block devices with 4k sectors imply the MBR sectors are also 4k instead
of regular 512B. Avoid hard-coding the 512B sector size and isntead read
the current block device sector size from it, and if the sector size is
larger than 512B, use the block device sector size.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>