staging: iio: set proper supply name to devm_regulator_get()

The name passed to devm_regulator_get() should match the name of the
supply as specified in the device datasheet. This makes it clear what
power supply is being referred to in case of presence of other
regulators.

Currently, the supply name specified on the affected devices is 'vcc'.
Use lowercase version of the datasheet name to specify the supply
voltage.

Suggested-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>