clk: ti: Update for of_clk_get_parent_count() returning unsigned int

Change the types here to unsigned int instead of int and update
the checks for == 0 instead < 1 to be more explicit about what's
going on now that of_clk_get_parent_count() has changed return
types.

Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>