spi: altera: Make SPI_ALTERA_CORE invisible

The SPI_ALTERA_CORE config symbol controls compilation of the Altera SPI
Controller core code.  It is already selected by all of its users, so
there is no reason to make it visible, unless compile-testing.

Fixes: b0c3d935 ("spi: altera: separate core code from platform code")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/f0cb8e66baba4506db6f42fca74dc51b76883507.1619534253.git.geert+renesas@glider.be


Signed-off-by: Mark Brown <broonie@kernel.org>