mlxsw: spectrum_dpipe: Add missing error path

In case devlink_dpipe_entry_ctx_prepare() failed, release RTNL that was
previously taken and free the memory allocated by
mlxsw_sp_erif_entry_prepare().

Fixes: 2ba5999f

 ("mlxsw: spectrum: Add Support for erif table entries access")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>