serial: s5p: Fix clk_get_by_index() error code check

clk_get_by_index() returns negative number on error. Assigning it to
unsigned int makes the subsequent "ret < 0" check always false, leading
in turn to possible unhandled errors. Change 'ret' variable type to
signed int so the code checks and handles clk_get_by_index() return code
properly.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Fixes: cf75cdf9 ("serial: s5p: use clock api to get clock rate")
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>