rsi: Add support for antenna selection

RSI 9113 device supports single antenna for tx and rx. Support for using
external is added. This can be configured from user space using iw.

Signed-off-by: Prameela Rani Garnepudi <prameela.j04cs@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>