s390/cio: exclude subchannels with no parent from pseudo check

ccw console is created early in start_kernel and used before css is
initialized or ccw console subchannel is registered. Until then console
subchannel does not have a parent. For that reason assume subchannels
with no parent are not pseudo subchannels. This fixes the following
kasan finding:

BUG: KASAN: global-out-of-bounds in sch_is_pseudo_sch+0x8e/0x98
Read of size 8 at addr 00000000000005e8 by task swapper/0/0

CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.3.0-rc8-07370-g6ac43dd12538 #2
Hardware name: IBM 2964 NC9 702 (z/VM 6.4.0)
Call Trace:
([<000000000012cd76>] show_stack+0x14e/0x1e0)
 [<0000000001f7fb44>] dump_stack+0x1a4/0x1f8
 [<00000000007d7afc>] print_address_description+0x64/0x3c8
 [<00000000007d75f6>] __kasan_report+0x14e/0x180
 [<00000000018a2986>] sch_is_pseudo_sch+0x8e/0x98
 [<000000000189b950>] cio_enable_subchannel+0x1d0/0x510
 [<00000000018cac7c>] ccw_device_recognition+0x12c/0x188
 [<0000000002ceb1a8>] ccw_device_enable_console+0x138/0x340
 [<0000000002cf1cbe>] con3215_init+0x25e/0x300
 [<0000000002c8770a>] console_init+0x68a/0x9b8
 [<0000000002c6a3d6>] start_kernel+0x4fe/0x728
 [<0000000000100070>] startup_continue+0x70/0xd0

Cc: stable@vger.kernel.org
Reviewed-by: Sebastian Ott <sebott@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>