pasemi_mac: pass in count of buffers to replenish rx ring with

Refactor replenish_rx_ring to take an argument for how many entries to
fill. Since it's normally available from where it's called anyway, this
is just simpler. It also removes the awkward logic to try to figure out
if we're filling for the first time or not.
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>