sdcard: support thingplus-rp2040

The instances of SPI interfaces SPI0, SPI1, ... are
pointers to SPI types on rp2040 machines, rather plain
values like other machines.

This needs restructuring the sdcard API to take a pointer
to the SPI interface rather the SPI type itself.

Removed waitserial() since it needs modem control lines
not available on most boards

examples/sdcard/tinyfs/feather-m4.go

@@ -8,7 +8,7 @@ import (
 )
 
 func init() {
-	spi = machine.SPI0
+	spi = &machine.SPI0
 	sckPin = machine.SPI0_SCK_PIN
 	sdoPin = machine.SPI0_SDO_PIN
 	sdiPin = machine.SPI0_SDI_PIN