refactor: use *SPI everywhere to make consistant for implementations.

Fixes #4663 "in reverse" by making SPI a pointer everywhere, as discussed
in the comments.

Signed-off-by: deadprogram <ron@hybridgroup.com>

src/machine/board_nucleol031k6.go

@@ -86,11 +86,11 @@ var (
 	I2C0 = I2C1
 
 	// SPI
-	SPI0 = SPI{
+	SPI0 = &SPI{
 		Bus:             stm32.SPI1,
 		AltFuncSelector: 0,
 	}
-	SPI1 = &SPI0
+	SPI1 = SPI0
 )
 
 func init() {