Bluetooth: controller: remove dup code in packet_rx_reserve_get()

The calculation in packet_rx_reserve_get() was already handled by
packet_rx_acquired_count_get().  So, let's use that code instead
and remove the duplication.

Change-Id: Ic76f70f1e78bebc74f5bef36cd92a3c332e489e9
Signed-off-by: Michael Scott <michael.scott@linaro.org>