serial: jsm: Replace magic value with the proper define

The changed function flushes the tx UART and the '4' corresponds to the
UART_FCR_CLEAR_XMIT value. This commit replaces the magic number with
this define.

Signed-off-by: Konrad Zapalowicz <bergo.torino@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>