LinuxKPI: string_choices.h: use ternary operator

Switch from using if () else to a direct return (?:) code.
No functional changes.

Suggested by: kib (D55029)
Sponsored by: The FreeBSD Foundation
Reviewed by: emaste (before removing more () as suggested by him)
Differential Revision: https://reviews.freebsd.org/D55088

(cherry picked from commit 8df7af9c9ecf7fc0b1c664f3d95893a9fcc16fcd)