cxgb4: remove cast when saving IPv4 partial checksum

The checksum field in IPv4 header is in __sum16 and ip_fast_csum()
also returns __sum16. So, no need to cast it to u16.

Fixes following sparse warning:
sge.c:1539:47: warning: cast from restricted __sum16
sge.c:1539:44: warning: incorrect type in assignment (different base types)
sge.c:1539:44:    expected restricted __sum16 [usertype] check
sge.c:1539:44:    got unsigned short [usertype]

Fixes: d0a1299c ("cxgb4: add support for vxlan segmentation offload")
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>