net: pkt: Add explicit casts to uint32_t when bitshifting uint8_t val

Cast uint8_t variable to uint32_t explicitly to avoid implicit cast to
int, and thus potentially undefined behavior, reported by UBSAN:

  net_pkt.c:1946:17: runtime error: left shift of 239 by 24 places
  cannot be represented in type 'int'

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>