mlxsw: spectrum_nve: Breakout common code to a common function

The configuration of a VXLAN tunnel in Spectrum-1 and Spectrum-2 is
largely the same. To avoid code duplication, breakout the common parts
to a common function that can be invoked from the ASIC-specific code.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>