Bluetooth: Controller: Fix HCI command buffer allocation failure

Fix HCI command buffer allocation failure, that can cause
loss of Host Number of Completed Packets command.

Fail by rejecting the HCI Host Buffer Size command if the
required number of HCI command buffers are not allocated in
the Controller implementation.

When Controller to Host data flow control is supported in
the Controller only build, ensure that BT_BUF_CMD_TX_COUNT
is greater than or equal to (BT_BUF_RX_COUNT + Ncmd),
where Ncmd is supported maximum Num_HCI_Command_Packets in
the Controller implementation.

Relates to commit 81614307 ("Bluetooth: Add workaround
for no command buffer available")'.

Relates to commit 297f4f48 ("Bluetooth: Split HCI
command & event buffers to two pools").

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