clk: st: Remove impossible check for of_clk_get_parent_count() < 0

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

Cc: Gabriel Fernandez <gabriel.fernandez@st.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>