Let the flash context directly point to the used master

We used to have a pointer to a full `registered_master` struct in
our flash context. Beside the used master, this contained a bit
mask of supported buses. Oddly convenient, this bit mask invited
to bypass the chip driver and break the abstraction. It allowed
to place bus-specific details virtually anywhere in flashprog,
making it harder to find a good place for them.

So, get rid of the `buses_supported` bit mask by pointing directly
to the master. Only the chip driver will implicitly know which type
of master is used.

Change-Id: I9ce13d8df0e7ccc67519d888dd9cb2e2ff8d6682
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/72533
diff --git a/jlink_spi.c b/jlink_spi.c
index 61338b6..7534cbb 100644
--- a/jlink_spi.c
+++ b/jlink_spi.c
@@ -109,7 +109,7 @@
 {
 	uint32_t length;
 	uint8_t *buffer;
-	struct jlink_spi_data *jlink_data = flash->mst->spi.data;
+	struct jlink_spi_data *jlink_data = flash->mst.spi->data;
 
 	length = writecnt + readcnt;