via-velocity: move residual free rx descriptors count register update

Updates of the RBRDU have two different meanings depending on their
context:
1. the receiving process has not started - the value which is written
   into the RBRDU register is supposed to be the free rx descriptors
   count (rounded to a multiple of 4)
2. the receiving process is running - the value increments the count
   above (sic)

The update is currently issued deep inside the rx replenish chain (see
velocity_give_many_rx_descs).

Let's propagate enough information to the caller so that the rx
replenish functions do not depend on hardware any more.

It is needed to perform the Rx/Tx buffers housekeeping when MTU changes.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>