clk: exynos: Fix always true test

clk: exynos: Fix always true test

In exynos7420_peric1_get_rate the variable ret is declared as an
'unsigned int' but is then used to receive the return value of
clk_get_by_index which returns an int. The value of ret is then tested
for being less than 0 which will always fail for an unsigned variable.
Fix this by declaring ret as an 'int' so that the test for the error
condition is valid.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>