mlxsw: Add bus capability flag

The mlxsw core infrastructure currently assumes that communication with
the ASIC is always possible using Ethernet management datagrams (EMADs),
but this is only possible when the PCI bus is used.

The bus capability flag is added to indicate EMAD support and make core
initialize EMAD communication only when it's set. Otherwise, register
access is done using command interface.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>