Bluetooth: Controller: Fix regression in BT_RX_STACK_SIZE use

Fix regression in BT_RX_STACK_SIZE use due to
commit b9172861 ("Bluetooth: host: remove
`CONFIG_BT_RECV_BLOCKING`").

Profiling of Controller Rx thread has been mentioned in
commit 586ba9fd ("Bluetooth: Controller: Increase
BT_RX_STACK_SIZE for BT_HCI_RAW enabled").

Also, move the TX_STACK_SIZE value as default in Kconfig
from the explicit value being set in hci_ipc sample.

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