clk: vt8500: fix sign of possible PLL values

With unsigned values underflow in loops can occur resulting in
theoretically infinite loops.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2038576



Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>