spi25: Move 4BA preparations into spi_prepare_4ba() hook

These preparations are specific to 4BA SPI chips and don't have to
clutter `flashprog.c`.

Change-Id: I842244c57e575f93b9c505e16f1f20c7afd23733
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/72517
4 files changed