flash: spi_nor: build fails when CONFIG_SPI_NOR_SFDP_RUNTIME is enabled

The spi_nor driver fails to build when the CONFIG_SPI_NOR_SFDP_RUNTIME option is enabled.

Here is the build error message:
```
drivers/flash/spi_nor.c: In function 'spi_nor_configure':
drivers/flash/spi_nor.c:1071:6: error: 'cfg' undeclared (first use in this function)
  if (cfg->has_lock != 0) {
```
