mlxsw: spectrum_switchdev: Check success of FDB add operation

Until now, we assumed that in case of error when adding FDB entries, the
write operation will fail, but this is not the case. Instead, we need to
check that the number of entries reported in the response is equal to
the number of entries specified in the request.

Fixes: 56ade8fe

 ("mlxsw: spectrum: Add initial support for Spectrum ASIC")
Reported-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Shalom Toledo <shalomt@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>