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