regulator: tps65217: Fix build warnings

rdev_get_id() returns an int. Convert rid to type int to avoid the
following warnings:

drivers/regulator/tps65217-regulator.c:73:10: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
drivers/regulator/tps65217-regulator.c:87:10: 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>