mailbox: platform-mhu: Remove redundant dev_err()

There is no need to call the dev_err() function directly to print a custom
message when handling an error from platform_get_irq() function as
it is going to display an appropriate error message in case of a failure.

Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>