SPI: zynq: Remove data memory barrier

This data memory barrier was previously added as a workaround
to the clock domain crossing issue. With the solution of using
transmit count without relying on the status flag, dmb()
is no longer required. Hence removed it.

Signed-off-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>