gre/ipip: use be16 variants of netlink functions

encap.sport and encap.dport are __be16, use nla_{get,put}_be16 instead
of nla_{get,put}_u16.

Fixes the sparse warnings:

warning: incorrect type in assignment (different base types)
   expected restricted __be32 [addressable] [usertype] o_key
   got restricted __be16 [addressable] [usertype] i_flags
warning: incorrect type in assignment (different base types)
   expected restricted __be16 [usertype] sport
   got unsigned short
warning: incorrect type in assignment (different base types)
   expected restricted __be16 [usertype] dport
   got unsigned short
warning: incorrect type in argument 3 (different base types)
   expected unsigned short [unsigned] [usertype] value
   got restricted __be16 [usertype] sport
warning: incorrect type in argument 3 (different base types)
   expected unsigned short [unsigned] [usertype] value
   got restricted __be16 [usertype] dport

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>