bluetooth: ascs: Fix NULL pointer dereference in ascs_ase_read

"The (ATT attribute read) callback can also be used locally to
read the contents of the attribute in which case no connection
will be set."

This means that callback may be called with NULL conn. Since
for ascs_ase_read NULL conn object have no meaning just return
error.

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>