clk: st: fix handling result of of_property_count_strings

The function can return negative value.

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

[1]: http://permalink.gmane.org/gmane.linux.kernel/2046107



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