Bluetooth: Controller: Fix Tx Ack FIFO index corruption under race

Fix Tx Ack FIFO's first index being advanced beyond a
recorded ack_last value in a node_rx when under race
between ll_rx_get() being pre-empted while executing the
`tx_cmplt_get()` and a call to `ll_rx_put()` in an
interrupt service routine.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>