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.

Signed-off-by: Clément Le Goffic <clement.legoffic@foss.st.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202310191831.MLwx1c6x-lkp@intel.com/
Fixes: fee68164 ("spi: stm32: disable device mode with st,stm32f4-spi compatible")
Link: https://patch.msgid.link/20250616-spi-upstream-v1-2-7e8593f3f75d@foss.st.com


Signed-off-by: Mark Brown <broonie@kernel.org>