shared/att: Fix signed write without transparent signing
shared/att: Fix signed write without transparent signing

If local key wasn't set by bt_att_set_local_key() assume that signing
is done by bt_att_send() caller ie. gattrib code.

mapfau/bluez,silent-snowman/bluez,mapfau/bluez,mapfau/bluez,ComputeCycles/bluez,silent-snowman/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,pkarasev3/bluez,ComputeCycles/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,pkarasev3/bluez,ComputeCycles/bluez,mapfau/bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez