via-velocity: Give RX descriptors to the NIC later on open or MTU change

velocity_open() calls velocity_give_many_rx_descs(), which gives RX
descriptors to the NIC, before installing an interrupt handler or
calling velocity_init_registers().  I think this is very unsafe and it
appears to explain the bug report <http://bugs.debian.org/508527

>.

On MTU change, velocity_give_many_rx_descs() is again called before
velocity_init_registers().  I'm not sure whether this is unsafe but
it does look wrong.

Therefore, move the calls to velocity_give_many_rx_descs() after
request_irq() and velocity_init_registers().

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Tested-by: Jan Ceuleers <jan.ceuleers@computer.org>
Signed-off-by: David S. Miller <davem@davemloft.net>