Makefile: 'M=' is preferred to 'KBUILD_EXTMOD=' for external modules.

I should have used 'M=' in (e93c53ce05 "Makefile: SUBDIRS ->
KBUILD_EXTMOD") originally.

See:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/kbuild/modules.rst?h=v5.3#n77
Signed-off-by: Shaun Ruffell <sruffell@sruffell.net>

Makefile

@@ -53,7 +53,7 @@ INST_HEADERS:=kernel.h user.h fasthdlc.h wctdm_user.h dahdi_config.h
 
 DAHDI_BUILD_ALL:=m
 
-KMAKE=+$(MAKE) -C $(KSRC) KBUILD_EXTMOD=$(PWD)/drivers/dahdi DAHDI_INCLUDE=$(PWD)/include DAHDI_MODULES_EXTRA="$(DAHDI_MODULES_EXTRA)" HOTPLUG_FIRMWARE=$(HOTPLUG_FIRMWARE)
+KMAKE=+$(MAKE) -C $(KSRC) M=$(PWD)/drivers/dahdi DAHDI_INCLUDE=$(PWD)/include DAHDI_MODULES_EXTRA="$(DAHDI_MODULES_EXTRA)" HOTPLUG_FIRMWARE=$(HOTPLUG_FIRMWARE)
 
 ROOT_PREFIX:=
 

drivers/dahdi/datamods/Makefile

@@ -8,7 +8,7 @@ PWD=$(shell pwd)
 
 MODULESO:=$(MODULES:%=%.o)
 MODULESKO:=$(MODULES:%=%.ko)
-KMAKE = $(MAKE) -C $(KSRC) KBUILD_EXTMOD=$(PWD)
+KMAKE = $(MAKE) -C $(KSRC) M=$(PWD)
 KMAKE_INST = $(KMAKE) \
   INSTALL_MOD_PATH=$(INSTALL_PREFIX) INSTALL_MOD_DIR=misc modules_install