thunderbolt: Add casts to prevent endianness warnings

Thunderbolt packets are big endian. Cast pkg->buffer to __be32* when
accessing the checksum.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>