typhoon: wait for RX mode commands to finish

When adding VLAN devices, we can get several calls to
typhoon_set_rx_mode() in quick succession. Because we didn't wait for
the commands to complete, we could run out of command descriptors and
fail to set the RX mode.

Signed-off-by: David Dillow <dave@thedillows.org>
Signed-off-by: David S. Miller <davem@davemloft.net>