net: mvpp2: prevent buffer overflow in mvpp22_rss_ctx()

The "rss_context" variable comes from the user via  ethtool_get_rxfh().
It can be any u32 value except zero.  Eventually it gets passed to
mvpp22_rss_ctx() and if it is over MVPP22_N_RSS_TABLES (8) then it
results in an array overflow.

Fixes: 895586d5

 ("net: mvpp2: cls: Use RSS contexts to handle RSS tables")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>