drivers/spi/spi_xlnx_axi_quadspi: Build fix w CONFIG_SPI_ASYNC

Fix a build error when CONFIG_SPI_ASYNC is set.
The issue was detected by CI

```
spi_xlnx_axi_quadspi.c: In function 'xlnx_quadspi_isr':
spi_xlnx_axi_quadspi.c:489:21: error: 'ctx' undeclared
  489 |                 if (ctx->asynchronous) {
      |                     ^~~
```

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>