octeontx2-pf: fix a buffer overflow in otx2_set_rxfh_context()

This function is called from ethtool_set_rxfh() and "*rss_context"
comes from the user.  Add some bounds checking to prevent memory
corruption.

Fixes: 81a43620 ("octeontx2-pf: Add RSS multi group support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>