regulator: vctrl: Use min() instead of doing it manually

Fix following coccicheck warning:
drivers/regulator/vctrl-regulator.c:188:15-17: WARNING opportunity for max()

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Link: https://lore.kernel.org/r/1647315190-16139-1-git-send-email-baihaowen@meizu.com


Signed-off-by: Mark Brown <broonie@kernel.org>