rsi: management frame descriptor preparation cleanup

Currently this descriptor is prepared with the help of __le16
pointer. This patch makes use of a structure to prepare the
descriptor in a cleaner way.

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>