disk: part_dos: correctly detect DOS PBR

The signature 0x55 0xAA in bytes 510 and 511 of the first sector can either
indicate a DOS partition table of the first sector of a FAT file system.

The current code tries to check if the partition table is valid by looking
at the boot indicator of the partition entries. But first of all it does
not count from 0 to 3 but only from 0 to 2. And second it misses to
increment the pointer for the partition entry.

If it is a FAT file system can be discovered by looking for the text 'FAT'
at offset 0x36 or 'FAT32' at offset 0x52. In a DOS PBR there are no
partition entries, so those bytes are undefined. Don't require the byte at
offset 0x1BE to differ from 0x00 and 0x80.

With the patch the logic is changed as follows:

If the partition table has either an invalid boot flag for any partition or
has no partition at all, check if the first sector is a DOS PBR by looking
at the FAT* signature.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>