firmware: meson_sm: fix to avoid potential NULL pointer dereference

of_match_device() may fail and returns a NULL pointer.

Fix this by checking the return value of of_match_device.

Fixes: 8cde3c21 ("firmware: meson_sm: Rework driver as a proper platform driver")
Signed-off-by: Zhang Shurong <zhang_shurong@foxmail.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/tencent_AA08AAA6C4F34D53ADCE962E188A879B8206@qq.com


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>