serial: Rename hw_stopped to old_hw_stopped & improve logic

serial: Rename hw_stopped to old_hw_stopped & improve logic

hw_stopped in uart_change_line_settings() stores old hw_stopped, thus
rename it appropriately.

Alter logic to check first if the hw_stopped was changed, and then pick
which function to call if it was because the logic is more obvious that
way.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230309080923.11778-8-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>