rtwn: Restore RF_ENV control type after initializing RF.

Both rwlwifi in Linux and rtwn in OpenBSD restore this register,
and the existing code was saving the value and not using it.

Reported by: -Wunused-but-set-variable
Reviewed by: imp
Obtained from: OpenBSD
Differential Revision: https://reviews.freebsd.org/D34838

(cherry picked from commit 50e3b6aa480b34e6eb87530d307db7ceba591a17)