bluetooth: sdp: cast from 'const x *' to 'struct x *' drops const qualifier

SonarQube reports: cast from 'const struct _snode *' to 'struct _snode *'
drops const qualifier. Fix it by removing the const.

Signed-off-by: Mark Wang <yichang.wang@nxp.com>
(cherry picked from commit 7982ee0df13b7f9e54595f799e32f38c464b0f9e)