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
MFC after: 3 days
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D45180