Change TftSpiBase displays to use SpiBus.Write instead of Exchange

[HakanL]

Fix for WildernessLabs/Meadow_Issues#826

Also changed API for ST7789 to have the reset pin optional (nullable) (it is optional when driving the display, so it should be nullable IMO)

[HakanL]

@ctacke what is your take on the proper way to do SpiBus.Exchange on a FT232H? It seems risky to have a spinning loop expecting data to be queued up after the full read is completed, the purpose of Exchange on SPI is to do a concurrent write and read. Also questionable if a large write (the display I'm testing with, ST7789, is sending 48k for a full update) would even have that much available space for a read queue?