atmel-samd: Update asf4 to include USB fix for SAMD51.

tannewt · dhalbert · commit 88c6d2349b0a · 2017-10-18T15:46:26.000-04:00
diff --git a/atmel-samd/asf4 b/atmel-samd/asf4
@@ -1 +1 @@
-Subproject commit 8dc86ee7657c92553e70803fdcef681a257f7e32
+Subproject commit bff3972abaeab79502bd7d0498b6ae81cc335e19
