staging: wilc1000: tcp_process: fix a build warning

This patch fixes build warning "flags is used uninitialized" when
TCP_ACK_FILTER is defined.

Fixes: 562ed3f1 ("staging/wilc1000: pass struct wilc to most linux_wlan.c functions")
Signed-off-by: Glen Lee <glen.lee@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>