in vt8237r_enable(), write function enables only to ISA bridge config space

in vt8237r_enable(), write function enables only to ISA bridge config space

vt8237r_enable() so far wrote the function enable values to the same
offset in the config space of every one of the vt8237's functions,
even though the register is located in the ISA bridge only.

Change-Id: I639586dc238132f5b8d2f320b794948718281b9c
Signed-off-by: Florian Zumbiehl <florz@florz.de>
Reviewed-on: http://review.coreboot.org/368
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>