rsi: RTS threshold configuration

Provision is added for configuring RTS threshold by sending
vap dynamic update frame to firmware.

Signed-off-by: Karun Eagalapati <karun256@gmail.com>
Signed-off-by: Amitkumar Karwar <amit.karwar@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>