examples/rp2/pio_uart_rx.py: Fix use of PIO constants.

Running the unmodified `pio_uart_rx.py` example by uploading the file and
importing it doesn't succeed, and instead emits a NameError at line 26.

Signed-off-by: Anson Mansfield <[email protected]>

Author: AJMansfield

examples/rp2/pio_uart_rx.py

@@ -23,7 +23,7 @@
 @asm_pio(
     autopush=True,
     push_thresh=8,
-    in_shiftdir=rp2.PIO.SHIFT_RIGHT,
+    in_shiftdir=PIO.SHIFT_RIGHT,
     fifo_join=PIO.JOIN_RX,
 )
 def uart_rx_mini():
@@ -42,7 +42,7 @@ def uart_rx_mini():
 
 
 @asm_pio(
-    in_shiftdir=rp2.PIO.SHIFT_RIGHT,
+    in_shiftdir=PIO.SHIFT_RIGHT,
 )
 def uart_rx():
     # fmt: off