clk: mxs: Index is always positive

Fix the following warnings when building with W=1 option:

drivers/clk/mxs/clk-imx23.c: In function 'mx23_clocks_init':
drivers/clk/mxs/clk-imx23.c:149:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/clk/mxs/clk-imx23.c:165:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
...

drivers/clk/mxs/clk-imx28.c: In function 'mx28_clocks_init':
drivers/clk/mxs/clk-imx28.c:227:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/clk/mxs/clk-imx28.c:244:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>