serial: stm32: fix transmit_chars when tx is stopped

Disables the tx irq  when the transmission is ended and updates stop_tx
conditions for code cleanup.

Fixes: 48a6092f ("serial: stm32-usart: Add STM32 USART Driver")
Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>