ALSA Makefile cleanup: use $(mod-subdirs)

ALSA Makefile cleanup: use $(mod-subdirs)

Some places were doing:

        subdir-$(CONFIG_FOO) += foo
        ifeq ($(CONFIG_FOO),y)
          subdir-m += foo
        fi

That can be expressed more easily as

        mod-subdirs := foo
        subdir-$(CONFIG_FOO) += foo