rt2800: tune TX_RTS_CFG config

Enable RTS frame retry fall-back and limit number of RTS retries to 7
what is default number of retries for small frames. As RTS/CTS is used
for TXOP protection, those settings prevent posting lots of RTS
frames when remote station do not response with CTS at the moment. After
sending 7 RTS's the HW will start back-off mechanism and after it will
start posing RTS again to get access to the medium.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>