spi: zynq_qspi: Clear the TX and RX threshold register

It's safe to clear the TX and RX threshold reg in controller
init, as previous SW might have change this threshold values.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>