drivers: serial: stm32: Fix possible issue with mixed enum

When building with LLVM we get the following error:

uart_stm32.c:272:9: error: implicit conversion from enumeration type
'enum uart_config_parity' to different enumeration type
'enum uart_config_flow_control' [-Werror,-Wenum-conversion]
        return UART_CFG_PARITY_NONE;
        ~~~~~~ ^~~~~~~~~~~~~~~~~~~~

We shouldn't be mixing parity and flow control enum's.  Use
UART_CFG_FLOW_CTRL_NONE instead or UART_CFG_PARITY_NONE.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>