samples: Bluetooth: hci_rpmsg/uart: Fix BT_CTLR_ISO_TX_BUFFER_SIZE

Restore back BT_CTLR_ISO_TX_BUFFER_SIZE to 251 after fix in
commit e460847b ("Bluetooth: host: don't fragment ISO
if len <= MTU").

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