part_efi: fix protective mbr struct allocation

The calloc() call was allocating space for the sizeof the struct
pointer rather than for the struct contents.
Besides, since this buffer is passed to mmc for writing and some
platforms may use cache, the legacy_mbr struct should be cache-aligned.

Signed-off-by: Hector Palacios <hector.palacios@digi.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>