simplefb: Remove impossible check for of_clk_get_parent_count() < 0

The check for < 0 is impossible now that
of_clk_get_parent_count() returns an unsigned int. Simplify the
code and update the types.

Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: <linux-fbdev@vger.kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>