fastboot: Change fastboot_buf_addr to an address

Given the name of this variable, it should be an address, not a
pointer. Update this, to make it easier to use with sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Dmitrii Merkurev <dimorinny@google.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com> # on vim3
diff --git a/cmd/fastboot.c b/cmd/fastboot.c
index c3c1923..792e83d 100644
--- a/cmd/fastboot.c
+++ b/cmd/fastboot.c
@@ -159,7 +159,7 @@
 		return CMD_RET_USAGE;
 	}
 
-	fastboot_init((void *)buf_addr, buf_size);
+	fastboot_init(buf_addr, buf_size);
 
 	if (!strcmp(argv[1], "udp"))
 		return do_fastboot_udp(argc, argv, buf_addr, buf_size);