stm32/usart: Add enable_/disable_error_interrupt
stm32/f4/memorymap.h: Fix ADC offsets
stm32/f4/adc: DMA can always be used with ADC2

I can't find any evidence in the manual to support this comment's claim.
Merging pull request #64 Various changes

Merge remote-tracking branch 'bgamari/master'