serial: mvebu-uart: fix return value check in mvebu_uart_clock_probe()

serial: mvebu-uart: fix return value check in mvebu_uart_clock_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: b7e2b5360f9b ("serial: mvebu-uart: implement UART clock driver for configuring UART base clock")
Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20220301075806.3950108-1-weiyongjun1@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>