net: cpsw: make TI_CPSW_PHY_SEL invisible

TI_CPSW_PHY_SEL depended on TI_CPSW and was selected by the latter. So
there is no reason to have this symbol visible.

A further optimisation would be to put the code for both symbols into a
single module which would allow to not export at least cpsw_phy_sel()
and simplify the module load process.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>