regulator: tps65218: Fix build warnings

rdev_get_id() returns an int. Convert rid to type int to avoid the
following warnings:
drivers/regulator/tps65218-regulator.c:132:10: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
drivers/regulator/tps65218-regulator.c:146:10: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Signed-off-by: Sachin Kamat <sachin.kamat@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>