bluetooth: host: fix RPMsg driver headroom configuration
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 <94a00d4c21e2b5f86e16c4df4b1ac94e543626d8@nordicsemi.no>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr