mlxsw: spectrum: Split a helper out of mlxsw_sp_netdevice_event()

Move the meat of mlxsw_sp_netdevice_event() to a separate function that
does just the validation. This separate helper will be possible to call
later for recursive ascent when validating attachment of a front panel port
to a bridge with uppers.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>