serial: stm32: extend TC timeout

Waiting 150us TC bit couldn't be enough.

If TFA lets 16 bits in USART fifo, we has to wait 16 times 87 us (time
of 10 bits (1 byte in most use cases) at a baud rate of 115200).

Fixes: b4dbc5d6 ("serial: stm32: Wait TC bit before performing initialization")

Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>