clk: sunxi: Use proper type for of_clk_get_parent_count() return value

The return type of of_clk_get_parent_count() is an unsigned int
now, so let's update the code here to be more explicit about the
range of values we can test for.

Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>