rsi: fix uninitialized descriptor pointer issue

This patch fixes the uninitialized descriptor pointer
issue in function rsi_send_internal_mgmt_frame().
Descriptor should point to start of the skb data.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 9a629faf ("rsi: immediate wakeup bit and priority for TX command packets")
Signed-off-by: Prameela Rani Garnepudi <prameela.j04cs@gmail.com>
Signed-off-by: Amitkumar Karwar <amit.karwar@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>