ALSA: intel_hdmi: remove redundant variable cfg_val

Variable cfg_val is being assigned but is never used hence it is
redundant and can be removed.

Cleans up clang warning:
warning: variable 'cfg_val' set but not used [-Wunused-but-set-variable]

[ Background info about val_bit field from alsa-devel ML thread:
 tiwai: Actually this made me wonder what is the definition of val_bit.
	It seems always 1 in the current code after the commit
	964ca808. Pierre?
 pbossart: This val_bit is only there for debug/test, it should be set
	to one by default and has nothing to do with the lpcm_id.
	This variable was set even in patches before upstream
	submission and was never needed, I guess it must be a 9-yr
	old issue. Good catch!
]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>