via-velocity: don't oops on MTU change (resend)

The VIA veloicty driver needs the following to allow changing MTU when down.
The buffer size needs to be computed when device is brought up, not when
device is initialized.  This also fixes a bug where the buffer size was
computed differently on change_mtu versus initial setting.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>