regulator: mpq7920: Check the correct variable in mpq7920_regulator_register()

There is a typo in the error checking.  We should be checking
"->rdev[i]" instead of just "->rdev".

Fixes: 6501c1f5

 ("regulator: mpq7920: add mpq7920 regulator driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20200113125805.xri6jqoxy2ldzqyg@kili.mountain


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