mlxsw: spectrum: Search for free LAD ID once

Currently, the function mlxsw_sp_lag_index_get() is called twice - first
as part of NETDEV_PRECHANGEUPPER event and later as part of
NETDEV_CHANGEUPPER. This function will be changed in the next patch. To
simplify the code, call it only once as part of NETDEV_CHANGEUPPER
event and set an error message using 'extack' in case of failure.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>