programmer: Smoothen register_spi_master() API

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

Change-Id: I0c753b3db050fb87d4bbe2301a7ead854f28456f
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/54066
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

buspirate_spi.c

@@ -655,7 +655,7 @@ int buspirate_spi_init(void)
 		return 1;
 	}
 
-	register_spi_master(&spi_master_buspirate);
+	register_spi_master(&spi_master_buspirate, NULL);
 
 	return 0;
 }