driver/serial : Fixed unwanted disabling of TX Interrupt
driver/serial : Fixed unwanted disabling of TX Interrupt

1. Do not disable Tx interrut if dev->xmit buffer is initially found empty
2. No bytes will be copied to DMA send VFIFO
3. Allow waiting processes to write to dev->xmit.head

pillip8282/TizenRT,jeongarmy/TizenRT,jeongchanKim/TizenRT,Samsung/TizenRT,junmin-kim/TizenRT,junmin-kim/TizenRT,sunghan-chang/TizenRT,jsdosa/TizenRT,jeongchanKim/TizenRT,jsdosa/TizenRT,jeongarmy/TizenRT,Samsung/TizenRT,an4967/TizenRT,Samsung/TizenRT,pillip8282/TizenRT,pillip8282/TizenRT,jeongarmy/TizenRT,jsdosa/TizenRT,Samsung/TizenRT,junmin-kim/TizenRT,an4967/TizenRT,jsdosa/TizenRT,jeongchanKim/TizenRT,an4967/TizenRT,pillip8282/TizenRT,jeongchanKim/TizenRT,junmin-kim/TizenRT,an4967/TizenRT,an4967/TizenRT,jeongarmy/TizenRT,pillip8282/TizenRT,sunghan-chang/TizenRT,Samsung/TizenRT,jsdosa/TizenRT,Samsung/TizenRT,pillip8282/TizenRT,Samsung/TizenRT,jsdosa/TizenRT,jeongarmy/TizenRT,sunghan-chang/TizenRT,jeongchanKim/TizenRT,jeongarmy/TizenRT,sunghan-chang/TizenRT,an4967/TizenRT,junmin-kim/TizenRT,junmin-kim/TizenRT,sunghan-chang/TizenRT,pillip8282/TizenRT,jsdosa/TizenRT,an4967/TizenRT,sunghan-chang/TizenRT,sunghan-chang/TizenRT,jeongchanKim/TizenRT,jeongarmy/TizenRT,jeongchanKim/TizenRT,junmin-kim/TizenRT