m68k: set proper major_num when specifying module param major_num

When calling register_blkdev() with specified major
device number, the return code is 0 on success.
So it seems not correct direct assign return code to
variable major_num in this case.
Tested-by: NMichael Schmitz <schmitzmic@gmail.com>
Reviewed-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: NChengguang Xu <cgxu519@gmx.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

arch/m68k/emu/nfblock.c

@@ -155,18 +155,22 @@ static int __init nfhd_init_one(int id, u32 blocks, u32 bsize)
 static int __init nfhd_init(void)
 {
 	u32 blocks, bsize;
+	int ret;
 	int i;
 
 	nfhd_id = nf_get_id("XHDI");
 	if (!nfhd_id)
 		return -ENODEV;
 
-	major_num = register_blkdev(major_num, "nfhd");
-	if (major_num <= 0) {
+	ret = register_blkdev(major_num, "nfhd");
+	if (ret < 0) {
 		pr_warn("nfhd: unable to get major number\n");
-		return major_num;
+		return ret;
 	}
 
+	if (!major_num)
+		major_num = ret;
+
 	for (i = NFHD_DEV_OFFSET; i < 24; i++) {
 		if (nfhd_get_capacity(i, 0, &blocks, &bsize))
 			continue;