serial: xilinx_uartps: unset STOPBRK when setting STARTBRK

Zynq UG585 states, in chapter B.33, for XUARTPS_CR_STARTBRK:
It can only be set if STPBRK (Stop transmitter break) is not high

This fixes tcsendbreak, which otherwise does not actually break.

Signed-Off-By: Julien Malik <julien.malik@unseenlabs.fr>
Link: https://lore.kernel.org/r/20230624210323.88455-1-julien.malik@unseenlabs.fr


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>