Keep unused native chip selects high (disabled)

This change prevents spi4teensy3 from interfering with other native SPI libraries using the native chip select signals.

Writing zeros to the SPI_MCR_PCSIS can cause other SPI devices to suddenly receive an active-low chip select.  Those 5 bits should always be written as ones when not using this feature.  (yes, the code in Teensyduino also do this... I'm updating it now)