hwmon: (iio_hwmon) Simplify attr.name generation in iio_hwmon_probe()

Since every call to devm_kasprintf() in the switch statement is mostly
the same, we can move all of the shared code outside and capture
differencies with two helper variables. No functional change intended.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>