xr_usb_serial_common: replace SUBDIRS with M in package recipe

The SUBDIRS variable has been removed in kernel 5.4, and was deprecated
since the beginnig of kernel git history in favour of M or KBUILD_EXTMOD.

Signed-off-by: David Bauer <mail@david-bauer.net>

libs/xr_usb_serial_common/Makefile

@@ -34,7 +34,7 @@ define Build/Compile
 	+$(MAKE) $(PKG_JOBS) -C "$(LINUX_DIR)" \
 		ARCH="$(LINUX_KARCH)" \
 		CROSS_COMPILE="$(TARGET_CROSS)" \
-		SUBDIRS="$(PKG_BUILD_DIR)/xr_usb_serial_common-1a" \
+		M="$(PKG_BUILD_DIR)/xr_usb_serial_common-1a" \
 		modules
 endef