It looks like the lpc11u34 ADC implementation is broken with regards to the resolution.

575659c broke it.

Given the definition of adc_res_t in periph_cpu.h: The check at
575659c#diff-50f14e4c5c396b6989e259ec75fac8c0R83
will always be true and the function will return -1.

I don't have the board to test and no time to fix it, just adding this as a reminder.