disable iimplicit-fallthrough warning inside asf4

.. there is an instance of it that looks like a "true positive", but it only
affects sdhc transfers that are not a multiple of 4 bytes, which I don't think
happens. (sd card blocks are always 512 bytes)  I can fix this in our
asf4 repo but that would mean this should be deferred until after adafruit#3384 is
merged, since that also touches asf4 very invasively by adding a whole new
chip family.

Author: jepler

ports/atmel-samd/Makefile

@@ -255,7 +255,7 @@ SRC_ASF += \
 	hal/src/hal_mci_sync.c \
 	hpl/sdhc/hpl_sdhc.c \
 
-$(BUILD)/asf4/$(CHIP_FAMILY)/hpl/sdhc/hpl_sdhc.o: CFLAGS += -Wno-cast-align
+$(BUILD)/asf4/$(CHIP_FAMILY)/hpl/sdhc/hpl_sdhc.o: CFLAGS += -Wno-cast-align -Wno-implicit-fallthrough
 endif
 
 $(BUILD)/asf4/$(CHIP_FAMILY)/hpl/sercom/hpl_sercom.o: CFLAGS += -Wno-maybe-uninitialized