staging: axis-fifo: fix return value check in axis_fifo_probe()

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

Fixes: 4a965c5f

 ("staging: add driver for Xilinx AXI-Stream FIFO v4.1 IP core")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>