net: ethernet: ti: convert to SPDX license identifiers

Replace textual license with SPDX-License-Identifier.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>