packets: Use WORDS_BIGENDIAN for struct geneve_opt.

The macro LITTLE_ENDIAN is a constant, not a test for endianness,
so it doesn't actually tell us anything about the machine.
WORDS_BIGENDIAN is both correct and defined by configure so it is
more portable.

Reported-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>