net: mvpp2: Prevent userspace from changing TX affinities

The mvpp2 driver can't cope at all with the TX affinities being
changed from userspace, and spit an endless stream of

[   91.779920] mvpp2 f4000000.ethernet eth2: wrong cpu on the end of Tx processing
[   91.779930] mvpp2 f4000000.ethernet eth2: wrong cpu on the end of Tx processing
[   91.780402] mvpp2 f4000000.ethernet eth2: wrong cpu on the end of Tx processing
[   91.780406] mvpp2 f4000000.ethernet eth2: wrong cpu on the end of Tx processing
[   91.780415] mvpp2 f4000000.ethernet eth2: wrong cpu on the end of Tx processing
[   91.780418] mvpp2 f4000000.ethernet eth2: wrong cpu on the end of Tx processing

rendering the box completely useless (I've measured around 600k
interrupts/s on a 8040 box) once irqbalance kicks in and start
doing its job.

Obviously, the driver was never designed with this in mind. So let's
work around the problem by preventing userspace from interacting
with these interrupts altogether.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>