audio-lnx: Add support to build sdm670 machine driver as module

When CONFIG_SND_SOC_EXT_CODEC/CONFIG_SND_SOC_INT_CODEC is set to m not y,
"ifdef CONFIG_SND_SOC_EXT_CODEC/CONFIG_SND_SOC_INT_CODEC"
would be false which would cause init functions as dummy functions.
Change to "if IS_ENABLED" and When CONFIG_SND_SOC_EXT_CODEC/
CONFIG_SND_SOC_INT_CODEC is set to y/m, both would set the
condition to true.
Change Makefile to compile machine driver for internal/external
codec as one module.

Change-Id: Ib72f95a24e8a4e657a1e9efc655f92a397b44d32
Signed-off-by: Meng Wang <mwang@codeaurora.org>