Update pio_spi.py

Missing import, see comments in raspberrypi#14

Author: Alasdair Allan

pio/pio_spi.py

@@ -1,3 +1,4 @@
+from rp2 import PIO, asm_pio
 from machine import Pin
 
 @rp2.asm_pio(out_shiftdir=0, autopull=True, pull_thresh=8, autopush=True, push_thresh=8, sideset_init=(rp2.PIO.OUT_LOW, rp2.PIO.OUT_HIGH), out_init=rp2.PIO.OUT_LOW)
@@ -45,4 +46,4 @@ def write_read_blocking(wdata):
         for b in wdata:
             self._sm.put(b << 24)
             rdata.append(self._sm.get() & 0xff)
-        return rdata
+        return rdata