p54pci: revise tx locking

This patch continues the effort which began with:
"[PATCH] p54pci: move tx cleanup into tasklet".

Thanks to these changes, p54pci's interrupt & tx
cleanup routines can be made lock-less.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>