Bluetooth: Fix code style error

checkpatch.pl flags the zero-length array members in struct
qca_dump_hdr:

    ERROR: Use C99 flexible arrays - see
    https://docs.kernel.org/process/deprecated.html#zero-length-and-one-element-arrays
    #3110: FILE: drivers/bluetooth/btusb.c:3110:
    + u8 data0[0];

Replace them with DECLARE_FLEX_ARRAY(), since C99 flexible array
members are not permitted inside unions or as the sole member of
a struct.

The struct layout is unchanged, and everything compiles with the
change.

Link: https://docs.kernel.org/process/deprecated.html#zero-length-and-one-element-arrays
Signed-off-by: Jeremy Dean <deaner92@yahoo.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>