superio/fintek/f71869ad: config struct should be const qualified

The 'superio_fintek_f71869ad_config' struct packed by devicetree.cb
should have its type declared with the 'const' qualifier.

Change-Id: Ieb86861ee821e77680cc4d0de202dbd7535b844d
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/6224
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>