via-velocity: Fix races on shared interrupts

This patch fixes two potential races in the velocity driver:

* Move the ACK and error handler to the interrupt handler. This fixes a
  potential race with shared interrupts when the other device interrupts
  before the NAPI poll handler has finished. As the velocity driver hasn't
  acked it's own interrupt, it will then steal the interrupt from the
  other device.

* Use spin_lock_irqsave in velocity_poll. In the current code, the
  interrupt handler will deadlock if e.g., the NAPI poll handler is
  executing when an interrupt (for another device) comes in since it
  tries to take the already held lock.

  Also unlock the spinlock only after enabling the interrupt in
  velocity_poll.

The error path is moved to the interrupt handler since this is where the
ISR is checked now.

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
Signed-off-by: Anders Grafstrom <anders.grafstrom@netinsight.net>
Signed-off-by: David S. Miller <davem@davemloft.net>