regulator: da9055: declare return type of inline function exclusively

da9055_regulator_dt_init does not declare return type and it cause
following build warning.

drivers/regulator/da9055-regulator.c:582:15: warning:
        return type defaults to ‘int’ [-Wreturn-type]
 static inline da9055_regulator_dt_init(struct platform_device *pdev,
               ^

Fix the warning by declare return type as int exclusively.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>