serial: stm32: Remove unused struct stm32_port txdone element

The txdone element is only ever assigned, once, and never used. Drop it.

Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20230223042252.95480-1-marex@denx.de


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