p54pci: move tx cleanup into tasklet

This patch moves the tx cleanup routines out of the critical
interrupt context and into the (previously known as rx) tasklet.

The main goal of this operation is to remove the extensive
usage of spin_lock_irqsaves in the generic p54common library.

The next step would be to modify p54usb to do the
rx processing inside a tasklet (just like usbnet).

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