net: ethernet: make inclusion relation clear for DSA and Ethernet headers

DSA is part of Ethernet and will utilize more Ethernet definitions for
more features support. So, it's proper to let DSA header include
Ethernet header with moving some DSA definiton from DSA header to
Ethernet header and adding DSA header including in c files using DSA
definition.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>