hwmon: (pmbus/tda38640) Use PMBUS_REGULATOR_ONE to declare regulator

If a chip only provides a single regulator, it should be named 'vout'
and not 'vout0'. Declare regulator using PMBUS_REGULATOR_ONE() to make
that happen.

Cc: Conor Dooley <conor@kernel.org>
Cc: Naresh Solanki <naresh.solanki@9elements.com>
Cc: Patrick Rudolph <patrick.rudolph@9elements.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240223-catnap-companion-c42fdd8ad110@spud


Signed-off-by: Guenter Roeck <linux@roeck-us.net>