rsi: separate function for data packet descriptor

Data packet descriptor preparation code is moved a separate
function.

Signed-off-by: Pavani Muthyala <pavani.muthyala@redpinesignals.com>
Signed-off-by: Amitkumar Karwar <amit.karwar@redpinesignals.com>
Signed-off-by: Prameela Rani Garnepudi <prameela.j04cs@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>