[stm32] fix adc

very hack-ish fix for when using multiple ADs, e.g. AD1 and AD2
Before this "fix" e.g. AD1_1 and AD2_1 had the same value (0).

Can probably be done more nicely...
[mcu_periph] NB_ADC only needed for lpc21
Merge pull request #822 from paparazzi/stm32_fix_adc

[stm32] fix adc

very hack-ish fix for when using multiple ADs, e.g. AD1 and AD2
Before this "fix" e.g. AD1_1 and AD2_1 had the same value (0).