samples: Bluetooth: hci_uart_3wire: Align Kconfigs with hci_uart

Align Kconfig values used with bt-ll-sw-split Controller as
same used in hci_uart sample.

Relates to commit 2c6306d0 ("Bluetooth: Controller:
BT_CTLR_ISO_TX_BUFFER_SIZE from BT_ISO_TX_MTU")'.

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