sh_eth: Fix promiscuous mode on chips without TSU

Currently net_device_ops::set_rx_mode is only implemented for
chips with a TSU (multiple address table).  However we do need
to turn the PRM (promiscuous) flag on and off for other chips.

- Remove the unlikely() from the TSU functions that we may safely
  call for chips without a TSU
- Make setting of the MCT flag conditional on the tsu capability flag
- Rename sh_eth_set_multicast_list() to sh_eth_set_rx_mode() and plumb
  it into both net_device_ops structures
- Remove the previously-unreachable branch in sh_eth_rx_mode() that
  would otherwise reset the flags to defaults for non-TSU chips

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>