rtwn: make sure RCR_APPFCS stays set in monitor mode / mode changes.

My previous commit meant that APPFCS wasn't enabled during monitor
mode and likely other corner cases.

Ensure it stays on at all times.

This, amusingly, fixes monitor mode in RTL8812AU/RTL8821AU - without it,
I don't see HT/VHT frames in monitor mode but I can still receive them
in normal STA mode.

Differential Revision: https://reviews.freebsd.org/D48112