SPI: zynq: Corrected SPI disable code

SPI disable was previously done by writing ~ZYNQ_SPI_ER_ENABLE_MASK,
which is 0xFFFFFFFE when in fact 0x0 should be written.
Corrected the same.

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