p54pci: prevent stuck rx-ring on slow system

This patch fixes an old problem, which - under certain
circumstances - could cause the device to become
unresponsive.

most of p54pci's rx-ring management is implemented in just
two distinct standalone functions. p54p_check_rx_ring takes
care of processing incoming data, while p54p_refill_rx_ring
tries to replenish all depleted communication buffers.

This has always worked fine on my fast machine, but
now I know there is a hidden race...

The most likely candidate here is ring_control->device_idx.
Quintin Pitts had already analyzed the culprit and posted
a patch back in Oct 2009. But sadly, no one's picked up on this.
( https://patchwork.kernel.org/patch/53079/

 [2 & 3] ).
This patch does the same way, except that it also prioritize
rx data processing, simply because tx routines *can* wait.

Reported-by: Sean Young <sean@mess.org>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=11386


Reported-by: Quintin Pitts <geek4linux@gmail.com>
Signed-off-by: Quintin Pitts <geek4linux@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>