Bluetooth: controller: nix #ifdefs from packet_rx_acquired_count_get()

This calculation can be used in packet_rx_reserve_get() where currently
the code is duplicated from packet_rx_acquired_count_get().
Let's allow use of packet_rx_acquired_count_get() regardless of whether
CONFIG_BLUETOOTH_CONTROLLER_DATA_LENGTH is enabled.

Change-Id: I613bde0a407f3caccabb22f369098575965e98ad
Signed-off-by: Michael Scott <michael.scott@linaro.org>