soc: qcom: cmd-db: Fix an error code in cmd_db_dev_probe()

The memremap() function doesn't return error pointers, it returns NULL.
This code is returning "ret = PTR_ERR(NULL);" which is success, but it
should return -ENOMEM.

Fixes: 312416d9 ("drivers: qcom: add command DB driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>