device property: fwnode->secondary may contain ERR_PTR(-ENODEV)

This fixes BUG triggered when fwnode->secondary is not NULL,
but has ERR_PTR(-ENODEV) instead.

BUG: unable to handle kernel paging request at ffffffffffffffed
IP: [<ffffffff81677b86>] __fwnode_property_read_string+0x26/0x160
PGD 200e067 PUD 2010067 PMD 0
Oops: 0000 [#1] SMP KASAN
Modules linked in: dwc3_pci(+) dwc3
CPU: 0 PID: 1138 Comm: modprobe Not tainted 4.5.0-rc5+ #61
task: ffff88015aaf5b00 ti: ffff88007b958000 task.ti: ffff88007b958000
RIP: 0010:[<ffffffff81677b86>]  [<ffffffff81677b86>] __fwnode_property_read_string+0x26/0x160
RSP: 0018:ffff88007b95eff8  EFLAGS: 00010246
RAX: fffffbfffffffffd RBX: ffffffffffffffed RCX: ffff88015999cd37
RDX: dffffc0000000000 RSI: ffffffff81e11bc0 RDI: ffffffffffffffed
RBP: ffff88007b95f020 R08: 0000000000000000 R09: 0000000000000000
R10: ffff88007b90f7cf R11: 0000000000000000 R12: ffff88007b95f0a0
R13: 00000000fffffffa R14: ffffffff81e11bc0 R15: ffff880159ea37a0
FS:  00007ff35f46c700(0000) GS:ffff88015b800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: ffffffffffffffed CR3: 000000007b8be000 CR4: 00000000001006f0
Stack:
 ffff88015999cd20 ffffffff81e11bc0 ffff88007b95f0a0 ffff88007b383dd8
 ffff880159ea37a0 ffff88007b95f048 ffffffff81677d03 ffff88007b952460
 ffffffff81e11bc0 ffff88007b95f0a0 ffff88007b95f070 ffffffff81677d40
Call Trace:
 [<ffffffff81677d03>] fwnode_property_read_string+0x43/0x50
 [<ffffffff81677d40>] device_property_read_string+0x30/0x40
...

Fixes: 362c0b30 (device property: Fallback to secondary fwnode if primary misses the property)
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>