staging: iio: ad7606: 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. The supply on this device
is called 'AVcc' while currently, the driver uses just 'vcc'.

Use 'avcc' to specify the supply voltage since it is custom to use the
lower-caps version of the datasheet name.

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