Fix RP2040 UART

It couldn't receive more than 32 bytes in while checking in_waiting
because in_waiting didn't turn interrupts back on correctly.

Fixes #6579

Author: tannewt

ports/raspberrypi/common-hal/busio/UART.c

@@ -311,7 +311,7 @@ uint32_t common_hal_busio_uart_rx_characters_available(busio_uart_obj_t *self) {
     // The UART only interrupts after a threshold so make sure to copy anything
     // out of its FIFO before measuring how many bytes we've received.
     _copy_into_ringbuf(&self->ringbuf, self->uart);
-    irq_set_enabled(self->uart_irq_id, false);
+    irq_set_enabled(self->uart_irq_id, true);
     return ringbuf_num_filled(&self->ringbuf);
 }