vendorcode/amd/agesa/f15tn: Remove extraneous bracket

Found an extra bracket that appears it should not be there.

Change-Id: I66b7967833afd25f12bd4eaaf6419a6ed3ad544b
Signed-off-by: Damien Zammit <damien@zamaudio.com>
Reviewed-on: http://review.coreboot.org/7515
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>