stlinkv3_spi: Move a declaration out of for-loop head

GCC 4.8 wants an explicit `-std=c99` or something for this to work. It
seems easier to keep the common declaration style.

Change-Id: Ic0819f82169df4d66cc949494229b0749c06e8f6
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/38034


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