ARM: SAMSUNG: Add tx_st_done variable

tx_st_done is required for checking the transmission status of SPI
channels with different fifo levels

Signed-off-by: Padmavathi Venna <padma.v@samsung.com>
Acked-by: Jassi Brar <jassisinghbrar@gmail.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>