programmer: Smoothen register_par_master API

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

Change-Id: I95bc92f6c54c5bcdac1c522ca87054aaffed0f40
Signed-off-by: Anastasia Klimchuk <aklm@chromium.org>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/54169
Original-Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/72201
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/programmer.h b/programmer.h
index f7ce7bc..7134b94 100644
--- a/programmer.h
+++ b/programmer.h
@@ -425,7 +425,7 @@
 	void (*chip_readn) (const struct flashctx *flash, uint8_t *buf, const chipaddr addr, size_t len);
 	void *data;
 };
-int register_par_master(const struct par_master *mst, const enum chipbustype buses);
+int register_par_master(const struct par_master *mst, const enum chipbustype buses, void *data);
 struct registered_master {
 	enum chipbustype buses_supported;
 	union {