net: phy: qcom: qca808x: add helper for checking for 1G only model

There are 2 versions of QCA808x, one 2.5G capable and one 1G capable.
Currently, this matter only in the .get_features call however, it will
be required for filling supported interface modes so lets add a helper
that can be reused.

Signed-off-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>