serial: move CFG_NS9750_UART to CONFIG_NS9750_UART

move also conditional compilation to Makefile

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/drivers/serial/Makefile b/drivers/serial/Makefile
index 3716974..2384735 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -27,7 +27,7 @@
 
 COBJS-$(CONFIG_ATMEL_USART) += atmel_usart.o
 COBJS-$(CONFIG_MCFUART) += mcfuart.o
-COBJS-y += ns9750_serial.o
+COBJS-$(CONFIG_NS9750_UART) += ns9750_serial.o
 COBJS-y += ns16550.o
 COBJS-$(CONFIG_DRIVER_S3C4510_UART) += s3c4510b_uart.o
 COBJS-y += serial.o