hw/char/sh_serial: fix break detection flag

sh_serial_receive_break() set bit 4 of the status register (sr) instead
of SH_SERIAL_FLAG_BRK (bit 3) in the flags field. The read path checks
flags, not sr, so break events were never visible to the guest.

Fixes: 63242a007a18 ("SH4: Serial controller improvement")
Reported-by: Pavee Hui <phui@google.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Yoshinori Sato <yoshinori.sato@nifty.com>
Message-ID: <20261004-issue-4560-v1-38-ccbef537cbeb@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>