serial: stm32: do not clear TC flag in async mode

The Transfer Complete flag (TC) is used to check if a transfer is
complete. This mechanism is used before suspending the UART module to
make sure that all data are sent before the suspend procedure.

The UART ISR clears this flag after completion of a async transfer which
causes a hang during UART device suspend setup.

There is just no need to clear this flag in ISR, it is cleared every
time we start a new async transfer.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>