drivers/adc: stm32: Use bitfield for multiple channels detection

For multiple channels detection, channels variable was compared with
the output of find_lsb_set which actually is a decimal number.
Since channel is a bitfield the comparison was not behaving as
expected (detecting several channels while only one channel was used).

Rework the code to use the already existing bitfield "index" for
the test.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>