stm32/uart: Clear overrun error flag after reading RX data register.
stm32/uart: Clear overrun error flag after reading RX data register.

On MCUs other than F4 the ORE (overrun error) flag needs to be cleared
independently of clearing RXNE, even though both are wired to trigger the
same RXNE IRQ.  In the case that an overrun occurred it's necessary to
explicitly clear the ORE flag or else the RXNE interrupt will keep firing.

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