drivers: adc: STM32: fix support for internal voltage reference source

Commit c045cbd3 added support for internal voltage reference source,
but in practice only the temp sensor is supported. Fix that.

Also change the code to keep the existing paths so that VREFINT and
TEMPSENSOR can be used at the same time.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>