rsi: update set_antenna command frame

TX command frame set_antenna is modified to use common
descriptor structure. Also it's subframe type is set.

Signed-off-by: Pavani Muthyala <pavani.muthyala@redpinesignals.com>
Signed-off-by: Amitkumar Karwar <amit.karwar@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>