part_dos: fix crash with big sector size

Apple iPod nanos have sector sizes of 2 or 4 KiB, which crashes U-Boot when it
tries to read the MBR into 512-byte buffer situated on stack. Instead use the
variable length arrays to be safe with any large sector size.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>