hwmon: (ads7828) Fix external VREF regulator handling

hwmon: (ads7828) Fix external VREF regulator handling

The driver currently has two issues with the external VREF regulator
handling in ads7828_probe():

1. All errors from devm_regulator_get_optional() are ignored, causing the
   driver to incorrectly fall back to internal VREF even for transient
   errors like -EPROBE_DEFER or genuine failures like -ENOMEM.

2. The external regulator is never enabled. The driver calls
   regulator_get_voltage() without first calling regulator_enable(),
   so the VREF pin may remain unpowered if the regulator is not
   configured as always-on.

Fix both issues by switching to devm_regulator_get_enable_read_voltage(),
which handles regulator get, enable, and voltage read in one call.
Only -ENODEV (no regulator specified in device tree) should trigger the
fallback to internal VREF. All other errors are propagated to the caller.

Fixes: a8ddfea09566 ("hwmon: (ads7828) Accept optional parameters from device tree")
Signed-off-by: Qingshuang Fu <fuqingshuang@kylinos.cn>
Link: https://lore.kernel.org/r/20260805061645.1331652-1-fffsqian@163.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>