net: ip: 6lo: Fix undefined behavior reported by UBSAN

htonl() and htons() take uint32_t/uint16_t as argument. Add the 'u' suffix
to constants to ensure the correct unsigned type is used and to avoid
undefined behavior if these functions are implemented as macros using
bit shifts.

Signed-off-by: Tim Pambor <tim.pambor@codewrights.de>