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: ad557eef77b ("serial: stm32: Wait TC bit before performing initialization")

Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Change-Id: Ie014d80c7039304fe05f8a3043408909c8bbcf15
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/u-boot/+/320045
Tested-by: Patrice CHOTARD <patrice.chotard@foss.st.com>
Reviewed-by: Patrice CHOTARD <patrice.chotard@foss.st.com>
Domain-Review: Patrice CHOTARD <patrice.chotard@foss.st.com>
ACI: CITOOLS <MDG-smet-aci-reviews@list.st.com>
ACI: CIBUILD <MDG-smet-aci-builds@list.st.com>