rsi: use appropriate interface for power save configuration

Power save request should be sent on station interface. Virtual
interface which is connected should be preferred. This patch
resolves device not entering power save problem in certain
situations

Signed-off-by: Amitkumar Karwar <amit.karwar@redpinesignals.com>
Signed-off-by: Siva Rebbagondla <siva.rebbagondla@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>