Bluetooth: ATT: Fix low throughput

ATT_PENDING_SENT does severely impact the throughput since multiple
packets no longer can be scheduled at same time, so instead of always
setting it regardless of the bearer/channel it is now only used for
EATT since that cannot set its own callbacks.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>