qcacld-3.0: Return proper error on request id mapping failure

When sending keepalive packets if there is failure in mapping
request id to pattern id in function -
hdd_map_req_id_to_pattern_id(), error code EINVAL is returned.
This error code is misleading and not sufficient to inform the
userspace that all available buffers are utilized and it should
stop sending keepalive packets.

Return proper error code if all available buffers are utilized
and no buffers are available to address any new request to send
keepalive packets.

Change-Id: Ie54299a0a7ff43a7044316d641d19ce12ac047c8
CRs-Fixed: 2445981