iio: adc: ad7192: handle regulator voltage error first

iio: adc: ad7192: handle regulator voltage error first

This change fixes a corner-case, where for a zero regulator value, the
driver would exit early, initializing the driver only partially.
The driver would be in an unknown state.

This change reworks the code to check regulator_voltage() return value
for negative (error) first, and return early. This is the more common
idiom.

Also, this change is removing the 'voltage_uv' variable and using the 'ret'
value directly. The only place where 'voltage_uv' is being used is to
compute the internal reference voltage, and the type of this variable is
'int' (same are for 'ret'). Using only 'ret' avoids having to assign it on
the error path.

Fixes: ab0afa65bbc7 ("staging: iio: adc: ad7192: fail probe on get_voltage")
Cc: Alexandru Tachici <alexandru.tachici@analog.com>
Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: <Stable@vger.kernel.org>