mailbox:armada-37xx-rwtm:remove duplicate print in armada_37xx_mbox_probe()

In this function,we don't need dev_err() message because when something
goes wrong,platform_get_irq() and devm_platform_ioremap_resource() have
print an error message itself, so we should remove duplicate dev_err().

Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>