clk: shmobile: Fix MSTP clock index

Use clkidx when registering MSTP clocks instead of loop counter
since the value is then used to access the specific clock index bit
in the mstp register.

The issue was introduced by the following commit:
f94859c2

 "clk: shmobile: Add MSTP clock support"

Changes in V2:
* none

Signed-off-by: Valentine Barshak <valentine.barshak@cogentembedded.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Mike Turquette <mturquette@linaro.org>