stlinkv3_spi: Avoid division by zero

Change-Id: I08c0612f3fea59add9bde2fb3cc5c4b5c3756516
Found-by: Coverity Scan #1412744
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/40653


Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>