Bluetooth: Rename SEND_STACK to TX_STACK_SIZE

For consistency with RX_STACK_SIZE and the rest of stack sizes, rename
to use the normal naming scheme.

Change-id: Ib8d484482466fa8d629e6329e07b927abdd6f598
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>