const'ify flashctx to align signatures with cros flashrom

The ChromiumOS flashrom fork has since const'ify flashctx
in a few places. This aligns the function signatures to
match with downstream to ease forward porting patches
out of downstream back into mainline flashrom.

This patch is minimum viable alignment and so feedback is
welcome.

Change-Id: Iff6dbda13cb0d941481c0d204b9c30895630fbd1
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/40324
Original-Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/70933
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/stlinkv3_spi.c b/stlinkv3_spi.c
index d2988b6..d56f305 100644
--- a/stlinkv3_spi.c
+++ b/stlinkv3_spi.c
@@ -323,7 +323,7 @@
 	return 0;
 }
 
-static int stlinkv3_spi_transmit(struct flashctx *flash,
+static int stlinkv3_spi_transmit(const struct flashctx *flash,
 				 unsigned int write_cnt,
 				 unsigned int read_cnt,
 				 const unsigned char *write_arr,