s5j/serial: fix compilation errors when UARTs are disabled

s5j/serial: fix compilation errors when UARTs are disabled

When CONFIG_S5J_UARTx is not set, compiler complains as following:

    CC:  chip/s5j_serial.c
    chip/s5j_serial.c: In function 'up_setup':
    chip/s5j_serial.c:1016:12: error: 'g_uart0priv' undeclared (first use in this function)
        priv = &g_uart0priv;
                ^
    chip/s5j_serial.c:1016:12: note: each undeclared identifier is reported only once for each function it appears in
    chip/s5j_serial.c:1019:12: error: 'g_uart1priv' undeclared (first use in this function)
        priv = &g_uart1priv;
                ^
    chip/s5j_serial.c:1022:12: error: 'g_uart2priv' undeclared (first use in this function)
        priv = &g_uart2priv;
                ^
    chip/s5j_serial.c:1025:12: error: 'g_uart3priv' undeclared (first use in this function)
        priv = &g_uart3priv;
                ^
    Makefile:175: recipe for target 's5j_serial.o' failed
    make[1]: *** [s5j_serial.o] Error 1

This commit fixes this.

Change-Id: Ifd56c89547071e1a96765e32cea984ac7234d181
Signed-off-by: Heesub Shin <heesub.shin@samsung.com>