Refactor SPI and fix write bug

* SPI write v2 was hanging in write due to an infinite loop
* SPI word write was not followed by a read back

The u8 and u16 write/read logic have been refactored into write_word and
read_word.

embassy-stm32/src/spi/mod.rs

@@ -12,6 +12,7 @@ use crate::gpio::Pin;
 pub enum Error {
     Framing,
     Crc,
+    ModeFault,
     Overrun,
 }