8250_men_mcb: fix error handling in read_uarts_available_from_reg()

If ioremap() fails, it returns NULL pointer, not ERR_PTR(), fix the
return value check and call release_mem_region() to release resource.

Fixes: c563831b ("8250_men_mcb: Make UART config auto configurable")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20230728085723.3195044-1-yangyingliang@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>