serial: liteuart: fix return value check in liteuart_probe()

serial: liteuart: fix return value check in liteuart_probe()

In case of error, the function devm_platform_get_and_ioremap_resource()
returns ERR_PTR() and never returns NULL. The NULL test in the return
value check should be replaced with IS_ERR().

Fixes: 1da81e5562fa ("drivers/tty/serial: add LiteUART driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20210305034929.3234352-1-weiyongjun1@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>