auxdisplay: ht16k33: Drop reference after LED registration

auxdisplay: ht16k33: Drop reference after LED registration

The reference count is bumped by device_get_named_child_node()
and never dropped. Since LED APIs do not require it to be
bumped by the user, drop the reference after LED registration.

[andy: rewritten the commit message and amended the change]

Fixes: c223d9c636ed ("auxdisplay: ht16k33: Add LED support")
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>