ARM: mach-shmobile: ap4evb: FSI clock use proper process for ak4642

Current AP4 FSI didn't use set_rate for ak4642,
and used dummy rate when init.
And FSI driver was modified to always call set_rate.

The user which are using FSI set_rate is only AP4 now.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>