Make sure we don't use an out-of-bound index for the per-port RSS
context array.

As of today, the global context creation in mvpp22_rss_context_create
will prevent us from reaching this case, but we should still make sure
we are using a sane value anyway.
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>