part: Allocate only one legacy_mbr buffer

Commit ff98cb90 ("part: extract MBR signature from partitions")
blindly switched allocated by ALLOC_CACHE_ALIGN_BUFFER buffer type from
"unsigned char" to "legacy_mbr" which caused allocation of size =
(typeof(legacy_mbr) * dev_desc->blksize) instead of just space enough
for "legacy_mbr" structure.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Peter Jones <pjones@redhat.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: Tom Rini <trini@konsulko.com>

disk/part_dos.c

@@ -90,7 +90,7 @@ static int test_block_type(unsigned char *buffer)
 static int part_test_dos(struct blk_desc *dev_desc)
 {
 #ifndef CONFIG_SPL_BUILD
-	ALLOC_CACHE_ALIGN_BUFFER(legacy_mbr, mbr, dev_desc->blksz);
+	ALLOC_CACHE_ALIGN_BUFFER(legacy_mbr, mbr, 1);
 
 	if (blk_dread(dev_desc, 0, 1, (ulong *)mbr) != 1)
 		return -1;