update mbed core, clearing serial errors properly

Wenn0101 · Wenn0101 · commit 553855f436c8 · 2022-03-16T20:38:50.000-06:00
diff --git a/cores/mbed-os b/cores/mbed-os
@@ -1 +1 @@
-Subproject commit 5e7ff58f3294033b015ea12fdf1b098109b0549d
+Subproject commit 22c8b6b1ee181d5efcc042d475f5c026545c4f84
