net: dsa: Provide proper address for dsa_api structure

The address of DSA API functions' structure available for usage from
the level of user's application (like for example in
samples/net/dsa/src) is stored in dsa context, not per DSA interface
device.

The struct dsa_context is defined once for all DSA (i.e. not master)
interfaces.

Signed-off-by: Lukasz Majewski <lukma@denx.de>