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 <sebastian.boe@nordicsemi.no>