programmer: Smoothen register_opaque_master() API

It was impossible to register a const struct opaque_master that would
point to dynamically allocated `data`. Fix that so that we won't
have to create more mutable globals.

Change-Id: Id3adb4cf04ae04dbe87ddb96f30871cb5f7c8ff0
Signed-off-by: Anastasia Klimchuk <aklm@chromium.org>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/54170
Original-Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
Original-Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/72202
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/linux_mtd.c b/linux_mtd.c
index 125de6b..6a9257f 100644
--- a/linux_mtd.c
+++ b/linux_mtd.c
@@ -426,7 +426,7 @@
 		goto linux_mtd_init_exit;
 	}
 
-	register_opaque_master(&linux_mtd_opaque_master);
+	register_opaque_master(&linux_mtd_opaque_master, NULL);
 
 	ret = 0;
 linux_mtd_init_exit: