clk: fix parent validation in __clk_set_parent()

The below commit introduced a bug in __clk_set_parent()
which could cause it to *skip* the parent validation
which makes sure the parent passed to the api is a valid
one.

    commit 7975059d


    Author: Rajendra Nayak <rnayak@ti.com>
    Date:   Wed Jun 6 14:41:31 2012 +0530

        clk: Allow late cache allocation for clk->parents

This was identified by the following compiler warning..

    drivers/clk/clk.c: In function '__clk_set_parent':
    drivers/clk/clk.c:1083:5: warning: 'i' may be used uninitialized in this function [-Wuninitialized]

.. as reported by Marc Kleine-Budde.

There were various options discussed on how to fix this, one
being initing 'i' to clk->num_parents, but the below approach
was found to be more appropriate as it also makes the 'parent
validation' code simpler to read.

Reported-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
Cc: stable@kernel.org