wil6210: Fix IP version indication for Tx csum offload

Bit DMA_CFG_DESC_TX_OFFLOAD_CFG_L3T_IPV4_POS should be set for IPv4
only. Don't set it for IPv6

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>