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>

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