tty: Char: mxser, use THRE for ASPP_OQUEUE ioctl

In moxa specific ASPP_OQUEUE ioctl command, they apparently want
only know whether there is space in transmitter hold register.

So switch UART_LSR_TEMT to UART_LSR_THRE in that specific case
according to the change in 1.14 moxa drivers.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>