memory: stm32_omm: Fix NULL vs IS_ERR() check in probe()

The platform_get_resource_byname() function returns NULL on error.  It
doesn't return error pointers.  Update the check to match.

Fixes: 8181d061 ("memory: Add STM32 Octo Memory Manager driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/a1645f1eedaa9b2ae62ac07feed0552eea75bc46.1746781081.git.dan.carpenter@linaro.org


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>