subs requires a lo register (#1402)

diff --git a/src/rp2_common/pico_platform/include/pico/platform.h b/src/rp2_common/pico_platform/include/pico/platform.h
index 217fbeb..61eb53a 100644
--- a/src/rp2_common/pico_platform/include/pico/platform.h
+++ b/src/rp2_common/pico_platform/include/pico/platform.h
@@ -504,7 +504,7 @@
     pico_default_asm (
         "1: subs %0, #3\n"
         "bcs 1b\n"
-        : "+r" (minimum_cycles) : : "memory"
+        : "+l" (minimum_cycles) : : "memory"
     );
 }