spi: stm32: fix sram pool free in probe error path

Add a test to check whether the sram_pool is NULL before freeing it.

Fixes: d17dd2f1 ("spi: stm32: use STM32 DMA with STM32 MDMA to enhance DDR use")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Clément Le Goffic <clement.legoffic@foss.st.com>
Link: https://patch.msgid.link/20250630-spi-fix-v2-1-4680939e2a3e@foss.st.com


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