misc: genwqe: fix return value check in genwqe_device_create()

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

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Frank Haverkamp <haver@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>