Bluetooth: OTS: Fix issue with callbacks not being set

The checks for callbacks in bt_ots_init did not correctly take the
ots_init->features.oacp into account for all callbacks, which
caused some issues.

Slightly optimized the check for ots->cb->obj_read by moving the check
and reducing the number of places the code calls oacp_read_proc_cb.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>