memory: stm32-fmc2-ebi: check regmap_read return value

Check regmap_read return value to avoid to use uninitialized local
variables.

Signed-off-by: Christophe Kerello <christophe.kerello@foss.st.com>
Link: https://lore.kernel.org/r/20240226101428.37791-3-christophe.kerello@foss.st.com


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