Keep unused native SPI chip selects high (disabled)

teensy3/avr_emulation.h

@@ -838,7 +838,7 @@ public:
 		}
 		ctar |= (SPI0_CTAR0 & SPI_CTAR_DBR);
 		update_ctar(ctar);
-		mcr = SPI_MCR_DCONF(0);
+		mcr = SPI_MCR_DCONF(0) | SPI_MCR_PCSIS(0x1F);
 		if (val & (1<<MSTR)) mcr |= SPI_MCR_MSTR;
 		if (val & (1<<SPE)) {
 			mcr &= ~(SPI_MCR_MDIS | SPI_MCR_HALT);