samples: Bluetooth: hci_rpmsg: Fix Rx Prio thread stack usage

Fix Rx Prio thread stack usage value to Kconfig default
value. After long duration use of hci_rpmsg in audio
application, remaining stack was 80 bytes, hence increase
back to 448 as set in the Kconfig defaults.

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