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.

BUG=b:185191942
TEST=builds

Change-Id: Id3adb4cf04ae04dbe87ddb96f30871cb5f7c8ff0
Signed-off-by: Anastasia Klimchuk <aklm@chromium.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/54170
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

ichspi.c

@@ -2046,7 +2046,7 @@ int ich_init_spi(void *spibar, enum ich_chipset ich_gen)
 			}
 			hwseq_data.size_comp1 = tmpi;
 
-			register_opaque_master(&opaque_master_ich_hwseq);
+			register_opaque_master(&opaque_master_ich_hwseq, NULL);
 		} else {
 			register_spi_master(&spi_master_ich9, NULL);
 		}