p54pci: fix bugs in p54p_check_tx_ring

Hans de Goede identified a bug in p54p_check_tx_ring:

there are two ring indices. 1 => tx data and 3 => tx management.
But the old code had a constant "1" and this resulted in spurious
dma unmapping failures.

Cc: stable@kernel.org
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=583623


Bug-Identified-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>