iwmc3200wifi: hardware does not support IP checksum

The iwmc3200wifi hardware doesn't support IP checksum. So mark the
skb->ip_summed to CHECKSUM_NONE instead of CHECKSUM_UNNECESSARY.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>