spi: stm32: Check for cfg availability in stm32_spi_probe

In the Linux kernel, the following vulnerability has been resolved:

spi: stm32: Check for cfg availability in stm32_spi_probe

The stm32_spi_probe function now includes a check to ensure that the pointer returned by of_device_get_match_data is not NULL before accessing its members. This resolves a warning where a potential NULL pointer dereference could occur when accessing cfg->has_device_mode.

Before accessing the 'has_device_mode' member, we verify that 'cfg' is not NULL. If 'cfg' is NULL, an error message is logged.

This change ensures that the driver does not attempt to access configuration data if it is not available, thus preventing a potential system crash due to a NULL pointer dereference.