LinuxKPI: add FIELD_PREP_CONST()

Add FIELD_PREP_CONST() like FIELD_PREP() without any extra checks likely
expected on this version in Linux. This is called by an updated wireless
driver.

Sposnored by: The FreeBSD Foundation
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D45180

(cherry picked from commit bb025df257386c5d1087b652e8ed951bceb89862)