net:gue.h:Fix shifting signed 32-bit value by 31 bits problem

Fix GUE_PFLAG_REMCSUM to use "U" cast to avoid shifting signed
32-bit value by 31 bits problem.

Signed-off-by: Vandana BN <bnvandana@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>