[nrf fromtree] bluetooth: host: fix RPMsg driver headroom configuration

Corrected configuration of BT_HCI_RESERVE for the RPMsg HCI driver.
This change fixes the following assert:
[net_buf_simple_headroom(buf) >= len] @ ZEPHYR_BASE/subsys/net/buf.c:881

Signed-off-by: Kamil Piszczek <[email protected]>

Author: kapi-no

subsys/bluetooth/host/Kconfig

@@ -113,6 +113,7 @@ config BT_HCI_RESERVE
 	int
 	default 0 if BT_H4
 	default 1 if BT_H5
+	default 1 if BT_RPMSG
 	default 1 if BT_SPI
 	default 1 if BT_STM32_IPM
 	default 1 if BT_USERCHAN