regulator: mpq7920: Fix Woverflow warning on conversion

Fix warning Woverflow on type conversion reported on x86.

Fixes: 6501c1f5 (regulator: mpq7920: add mpq7920 regulator driver)
Signed-off-by: Saravanan Sekar <sravanhome@gmail.com>
Link: https://lore.kernel.org/r/20200110102220.7163-1-sravanhome@gmail.com


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