drivers/misc/ti-st: Prefer tty_driver_flush_buffer

The tty core provides an interface for flushing the driver's
write buffer: tty_driver_flush_buffer(); prefer the provided
interface over open-coded. Open-coding can lead to errors
such as the duplicated call in the st_kim driver.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>