serial: stm32: remove infinite loop possibility in putchar function

Rework stm32_usart_console_putchar() function in order to anticipate
the case where the character can never be sent.

Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Link: https://lore.kernel.org/r/20220419085330.1178925-2-valentin.caron@foss.st.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>