Update spi-zynq-qspi.c

This fixes an issue on my system when not booting from QSPI. The 2-bit qspi.Config_reg[FIFO_WIDTH] field must always be 2'b11 (Xilinx UG585 v1.6 p1437). This driver was assuming these bits are already set which was not true in this case.

Thanks,
Scott