staging: wilc1000: Use common structs to parse ip packets

Use structs ethhdr, iphdr and tcphdr instead of manual parsing in
tcp_process.
This commit fix handling of ip packets containing options.
It also fixes the following sparse warning:

drivers/staging/wilc1000//wilc_wlan.c:201:19: warning: cast to restricted __be16

Signed-off-by: Thibaut Robert <thibaut.robert@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>