pasemi_mac: don't enable rx before there are buffers on the ring

Reorder initialization of the DMA channels and the interface. Before there
was a time window when the interface was enabled before DMA was enabled.
Also, now there will always be RX buffers available at the time the
MAC interface is enabled, to avoid temporary out-of-buffer errors for the
very first packets (on busy networks).
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>