serial: 8250_bcm7271: Fix return value check in brcmuart_probe()

serial: 8250_bcm7271: Fix return value check in brcmuart_probe()

In case of error, the function devm_ioremap() returns NULL
pointer not ERR_PTR(). The IS_ERR() test in the return value
check should be replaced with NULL test.

Fixes: f5eb8632b214 ("serial: 8250: Add new 8250-core based Broadcom STB driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Al Cooper <alcooperx@gmail.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20210329140659.1832950-1-weiyongjun1@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>