ASoC: sti: fix return value check in uni_player_parse_dt_audio_glue()

In case of error, the function syscon_regmap_lookup_by_phandle() returns
ERR_PTR() and never returns NULL. The NULL test in the return value check
should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Mark Brown <broonie@kernel.org>