MLK-10404-1 spi: spi-imx: use XCH mode even in DMA mode

To workaroud the TKT238285, the safe way is use XCH mode in SDMA
script to simulate as PIO mode which never report such issue. Meanwhile,
set tx threashold as 0. But this workaroud will bring performance impacted,
below performance data is collected by 'dd' with SPI-NOR flash on i.mx6dl
sabresd board:

  mode                   write data            read data
--PIO                     194KB/s              644KB/s
--DMA normal
  (SMC, tx_thresh=32)     222KB/s              1.4MB/s
--DMA(XCH, tx_thresh=0)   210KB/s              1.0MB/s

Signed-off-by: Robin Gong <b38343@freescale.com>
(cherry picked from commit 01be65fa5617aa192307ca38b6fc6128f3f0c3f7)