mailbox: arm_mhu_db: Remove redundant dev_err call in mhu_db_probe()

There is a error message within devm_ioremap_resource
already, so remove the dev_err call to avoid redundant
error message.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Qiheng Lin <linqiheng@huawei.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>