ASoC: Intel: Skylake: make function skl_clk_round_rate static

The function skl_clk_round_rate is local to the source and does not
need to be in global scope, so make it static.

Cleans up sparse warning:
sound/soc/intel/skylake/skl-ssp-clk.c:250:6: warning: symbol
'skl_clk_round_rate' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Acked-By: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>