p54: prevent speculative execution

p54: prevent speculative execution

CVE-2017-5753 (Spectre v1 Intel)

Since the queue value in function p54_conf_tx()
seems to be controllable by userspace and later on
conditionally (upon bound check) used to resolve
priv->qos_params, insert an observable speculation
barrier before its usage. This should prevent
observable speculation on that branch and avoid
kernel memory leak.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>