chipset_enable.c: Spell `BIOS` in uppercase

Intel document #336067 uses `BIOS Control` to refer to this register.

Change-Id: Ib66547b2b5d77658ab1925e4ad3acfe44e14843c
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/40857


Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Marcello Sylvester Bauer <sylv@sylv.io>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>