qcacld-3.0: Avoid info leak in spectral scan handler

In __spectral_scan_msg_handler(), payload section of input data is
type casted to driver internal structure spectral_scan_msg without
validating payload length which can lead to kernel info leak
if the payload length is less than size of spectral_scan_msg.

To fix this, avoid type-cast and return error if payload length is
less than size of spectral_scan_msg.

Change-Id: Ie7e74cc2cdcf8136582e81ffc3a088fd5a881dc9
CRs-Fixed: 2468493