spi: zynq_qspi: Clear the previous config register value.

Clear the old config register value read from previous
SW before writing the new qspi configuration values on
zynq_qspi_init_hw.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Acked-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>