libata: finally use __initconst in ata_parse_force_one()

Just six days after this FIXME was added seven years ago, Sam Ravnborg
added the missing feature (37c514e3 "Add missing init section
definitions"), though it ended up being called __initconst.

Let's use it; better late than never.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Tejun Heo <tj@kernel.org>