bluetooth: Fix cast in bt_buf_get_type
bluetooth: Fix cast in bt_buf_get_type

The function returns an enum, not a u8_t, so we should cast
appropriately to avoid an implicit conversion.

This construct was triggering this compilation error when compiling
with CXX:

/home/sebo/zephyr/include/bluetooth/buf.h:85:9: error: invalid
  conversion from ‘u8_t {aka unsigned char}’ to ‘bt_buf_type’
  [-fpermissive] return *(u8_t *)net_buf_user_data(buf);

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,mbolivar/zephyr,kraj/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyriot/zephyr,mbolivar/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,Vudentz/zephyr,ldts/zephyr,kraj/zephyr,Vudentz/zephyr,explora26/zephyr,ldts/zephyr,Vudentz/zephyr,ldts/zephyr,zephyriot/zephyr,mbolivar/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr,galak/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,galak/zephyr,galak/zephyr,punitvara/zephyr,nashif/zephyr,kraj/zephyr,ldts/zephyr,kraj/zephyr,zephyriot/zephyr,punitvara/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,explora26/zephyr,punitvara/zephyr,punitvara/zephyr,punitvara/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,explora26/zephyr,mbolivar/zephyr