spi: altera_spi: Do not abuse CONFIG namespace

The value CONFIG_ALTERA_SPI_IDLE_VAL is never re-defined by a board.
Rename this to ALTERA_SPI_IDLE_VAL.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>