mlx4: fix mlx4_en_set_rxfh()

mlx4_en_set_rxfh() can crash if no RSS indir table is provided.

While we are at it, allow RSS key to be changed with ethtool -X

Tested:

myhost:~# cat /proc/sys/net/core/netdev_rss_key
b6:89:91:f3:b2:c3:c2:90:11:e8:ce:45:e8:a9:9d:1c:f2:f6:d4:53:61:8b:26:3a:b3:9a:57:97:c3:b6:79:4d:2e:d9:66:5c:72:ed:b6:8e:c5:5d:4d:8c:22:67:30:ab:8a:6e:c3:6a

myhost:~# ethtool -x eth0
RX flow hash indirection table for eth0 with 8 RX ring(s):
    0:      0     1     2     3     4     5     6     7
RSS hash key:
b6:89:91:f3:b2:c3:c2:90:11:e8:ce:45:e8:a9:9d:1c:f2:f6:d4:53:61:8b:26:3a:b3:9a:57:97:c3:b6:79:4d:2e:d9:66:5c:72:ed:b6:8e

myhost:~# ethtool -X eth0 hkey \
03:0e:e2:43:fa:82:0e:73:14:2d:c0:68:21:9e:82:99:b9:84:d0:22:e2:b3:64:9f:4a:af:00:fa:cc:05:b4:4a:17:05:14:73:76:58:bd:2f

myhost:~# ethtool -x eth0
RX flow hash indirection table for eth0 with 8 RX ring(s):
    0:      0     1     2     3     4     5     6     7
RSS hash key:
03:0e:e2:43:fa:82:0e:73:14:2d:c0:68:21:9e:82:99:b9:84:d0:22:e2:b3:64:9f:4a:af:00:fa:cc:05:b4:4a:17:05:14:73:76:58:bd:2f

Reported-by: Ben Hutchings <ben@decadent.org.uk>
Fixes: b9d1ab7e ("mlx4: use netdev_rss_key_fill() helper")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>