disk: part_dos: Use the original allocation scheme for the SPL case

Since commit ff98cb90 ("part: extract MBR signature from partitions")
SPL boot on i.MX6 starts to fail:

U-Boot SPL 2017.09-00221-g0d6ab32 (Oct 02 2017 - 15:13:19)
Trying to boot from MMC1
(keep in loop)

Use the original allocation scheme for the SPL case, so that MX6 boards
can boot again.

This is a temporary solution to avoid the boot regression.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Rob Clark <robdclark@gmail.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>