rtwn_usb(4): fix Tx instability with RTL8192CU chipsets

• Fix data frames transmission via POWER_STATUS register setup -

it seems to be set by MACID_CONFIG firmware command, which was broken*
in r290439 and later disabled in r307529.

We can re-enable it later if / when firmware rate adaptation will be
ready; however, this step will be required anyway - for firmware-less
builds.

• Force RTS / CTS protection frame rate to CCK1 (this rate works fine

without any additional setup; no better workaround is known yet).

The problem was not observed on the channel 1 or with CCK1 rate enforced
('ifconfig wlan0 ucastrate 1' for 11 b/g; not possible for 11n networks
due to ifconfig(8) bug).

• I'm not sure if it works before r290439 because - AFAIR - I never seen

firmware rate adaptation working for 10-STABLE urtwn(4)
(It needs EN_BCN bit set and RSSI updates at least).

Tested with RTL8188CUS in STA mode
(in regular mode and with disabled MRR - DARFRC*8 is set to 0)

PR: 233949
MFC after: 2 weeks