disk: efi: Correct backing up the MBR boot code

In commit e163a931 ("cmd: gpt: backup boot code before writing MBR")
there was added the procedure for storing old boot code when doing "gpt
write". But instead of storing just backup code, the whole MBR was
stored, and only specific fields were replaced further, keeping
everything else intact. That's obviously not what we want.

Fix the code to actually store only old boot code and zero out
everything else. This fixes next testing case:

    => mmc write $loadaddr 0x0 0x7b
    => gpt write mmc 1 $partitions

In case when $loadaddr address and further memory contains 0xff, the
board was bricked (ROM-code probably didn't like partition entries that
were clobbered with 0xff). With this patch applied, commands above don't
brick the board.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Cc: Alejandro Hernandez <ajhernandez@ti.com>
Tested-by: Andy Shevchenko <andy.shevchenko@gmail.com>