rtwn: enable FCS in the recive config to work around truncated frames

I noticed that on RTL8812AU/RTL8821AU receiving VHT frames that
I'd occasionally see frames missing the last 4 bytes. I can
easily reproduce it with a ping sweep and fast (10ms) between frames.

There's also a report of an earlier NIC (RTL8188EU) doing the same
thing with HT frames but not with OFDM (11g) frames.

After a bunch of poking, it turns out a driver where things DID work
properly for the other report kept FCS enabled, and trimmed it from
the frame before pushing it up to the network layer.

I did the same and it also worked fine.

The other solution was to disable PHYSTATUS notifications, but then
we'd get no per packet RX notifications (RX rate, RSSI, etc.)

Locally tested:

• RTL8192EU, STA mode (HT)
• RTL8812AU, STA mode (HT, VHT)
• RTL8821AU, STA mode (HT, VHT)

Differential Revision: https://reviews.freebsd.org/D47775