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/usbblaster_spi.c b/usbblaster_spi.c
index e2a0a4f..46de86c 100644
--- a/usbblaster_spi.c
+++ b/usbblaster_spi.c
@@ -179,7 +179,7 @@
 }
 
 /* Returns 0 upon success, a negative number upon errors. */
-static int usbblaster_spi_send_command(struct flashctx *flash, unsigned int writecnt, unsigned int readcnt,
+static int usbblaster_spi_send_command(const struct flashctx *flash, unsigned int writecnt, unsigned int readcnt,
 				       const unsigned char *writearr, unsigned char *readarr)
 {
 	uint8_t cmd;