asoc: codecs: fix compilation error for wcd codec

The previous commit fb24ad8485a assigns obj-y with
msm_hdmi_codec_rx.o instead of appending, thus giving
compilation error as previously assigned object files
are removed.
Fix by appending obj-y with msm_hdmi_codec_rx.o instead
of assigning.

Change-Id: I7c5ea5627e2e1888e3fc0b53a6650ab64b9a29a8
Signed-off-by: Shiv Maliyappanahalli <smaliyap@codeaurora.org>