serial: core: Disable uart_start() on uart_remove_one_port()

While rebinding a uart device in a loop I noticed we may see a tx related
race on uart_remove_one_port():

uart_write from n_tty_write
n_tty_write from file_tty_write.constprop.0
file_tty_write.constprop.0 from vfs_write
vfs_write from ksys_write
ksys_write from ret_fast_syscall

Let's disallow tx on port->UPF_DEAD. This flag gets set before we start
tearing down the port in uart_remove_one_port().

Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20230419115423.59957-1-tony@atomide.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>