disk: part_dos.c: Add a PBR check when MBR checking fails

Bug: SDCard with a messed up partition but still has a FAT signature
intact is readable in Linux but unreadable in uboot with 'fatls'.

Fix: When partition info checking fails, there is no checking for a
FAT signature (DOS_PBR) which will fail 'fatls'. FAT signature checking
is done when no valid partition is found in partition table. If FAT
signature is found, the disk will be read as PBR and continue
processing.

Signed-off-by: Darwin Dingel <darwin.dingel@alliedtelesis.co.nz>