drivers: uart_stm32: fix ORE hogging the CPU

If data is received before an IRQ handler was set and enabled,
hardware sets the ORE flag. Once set, the IRQ routine starts
hogging the CPU until ORE is cleared. This change will clear
the flag when user code attempts to drain incoming data.

Signed-off-by: Kiril Zyapkov <k.zyapkov@allterco.com>