LinuxKPI: string_choices.h: use ternary operator

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

Suggested by: kib (D55029)
Sponosred by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: emaste (before removing more () as suggested by him)
Differential Revision: https://reviews.freebsd.org/D55088