mmc: renesas_sdhi: merge clk_{start,stop} functions to set_clock

renesas_sdhi_clk_start() and renesas_sdhi_clk_stop() are now only
called from renesas_sdhi_set_clock().  Merge them.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>