ethernet: Make OA_TC6 config symbol invisible

Commit aa58bec0 ("net: ethernet: oa_tc6: implement register
write operation") introduced a library that implements the OPEN Alliance
TC6 10BASE-T1x MAC-PHY Serial Interface protocol for supporting
10BASE-T1x MAC-PHYs.

There is no need to ask the user about enabling this library, as all
drivers that use it select the OA_TC6 symbol.  Hence make the symbol
invisible, unless when compile-testing.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/3b600550745af10ab7d7c3526353931c1d39f641.1733994552.git.geert+renesas@glider.be


Signed-off-by: Jakub Kicinski <kuba@kernel.org>