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>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/40324


Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>