rsi: eliminate superfluous NULL check

msg is dereferenced before checking against NULL, e.g.
when assigning pad_bytes.
Remove the superfluous check in function rsi_mgmt_pkt_to_core.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>