Allow to set other bits in spi.mode

Hello,

I try to drive WS2818B LEDs with that module. The problem is that my SPI is Idle-HIGH which causes problems. There's already an option in the kernel to set it Idle-LOW but it's not accepted by the python module:

https://lore.kernel.org/lkml/CAOMZO5DMw-ETS_ZZSTvksPZPi41w85x4o-VE8jHebDFJc14HTw@mail.gmail.com/T/

The bit is SPI_MOSI_IDLE_LOW
