mlxsw: core: Prevent trap group setting if driver does not support EMAD

Avoid trap group setting if driver is not capable of EMAD support.
For example, "mlxsw_minimal" driver works over I2C bus, overs which
EMADs cannot be sent.
Validation is performed by testing feature 'MLXSW_BUS_F_TXRX'.

Fixes: 74e0494d ("mlxsw: core: Move basic_trap_groups_set() call out of EMAD init code")
Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>