extmod/btstack/btstack.mk: Use -Wno-implicit-fallthrough, not =0.

In 2ae3c890bd, -Wimplicit-fallthrough=0 was
added to get the build to pass.  This option is equivalent to
-Wno-implicit-fallthrough, and the latter is compatible with clang (while
the former is not).

Fixes issue #7546.

Signed-off-by: Damien George <damien@micropython.org>

extmod/btstack/btstack.mk

@@ -67,7 +67,7 @@ endif
 LIB_SRC_C += $(SRC_BTSTACK)
 
 # Suppress some warnings.
-BTSTACK_WARNING_CFLAGS = -Wno-old-style-definition -Wno-unused-variable -Wno-unused-parameter -Wimplicit-fallthrough=0
+BTSTACK_WARNING_CFLAGS = -Wno-old-style-definition -Wno-unused-variable -Wno-unused-parameter -Wno-implicit-fallthrough
 ifneq ($(CC),clang)
 BTSTACK_WARNING_CFLAGS += -Wno-format
 endif