rsi: Fix NULL pointer dereference in kmalloc

kmalloc can fail in rsi_register_rates_channels but memcpy still attempts
to write to channels. The patch replaces these calls with kmemdup and
passes the error upstream.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>