Bluetooth: OTS: Update OTS to use correct reserve define

Update the L2CAP OTS implementation to use the correct reserve
defitition. By not using the correct definition the stack always
allocates a new segment for the packet, which puts more stress on the
stack buffers.
Update TX data size to be more intuitive by adding the overhead, instead
of subtracting it. This increases the default MTU of the implementation.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>