mailbox: bcm-flexrm-mailbox: Remove redundant dev_err call in flexrm_mbox_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: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>