i2c: smbus: Check for parent device before dereference

An I²C adapter might be instantiated without parent. In such case
there is no property can be retrieved. Skip SMBus alert setup when
this happens.

Fixes: a263a840 ("i2c: smbus: Use device_*() functions instead of of_*()")
Reported-by:  <syzbot+0591ccf54ee05344e4eb@syzkaller.appspotmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>