programmer.h: Convert anon union to anon struct

Convert the anon union of registered masters in the mst
field of the flashctx to a anon struct. If we are going
to dereference a pointer there in an undefined way we
should crash and not plow ahead with invalid memory.

The user of the registered_masters type is therefore
responsible for querying the buses_supported field before
attempting to dereference a ptr field in the anon struct.

BUG=b:175849641
TEST=`flashrom -p internal --flash-name`

Change-Id: I576967a8599b923c902e39f177f39146291cc242
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/50246
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Reviewed-by: Peter Marheine <pmarheine@chromium.org>
Reviewed-by: Daniel Campello <campello@chromium.org>
Reviewed-by: Sam McNally <sammc@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/spi.c b/spi.c
index aed2a92..aa245d7 100644
--- a/spi.c
+++ b/spi.c
@@ -133,7 +133,7 @@
 
 int register_spi_master(const struct spi_master *mst)
 {
-	struct registered_master rmst;
+	struct registered_master rmst = {0};
 
 	if (!mst->write_aai || !mst->write_256 || !mst->read || !mst->command ||
 	    !mst->multicommand ||