gre: remove duplicated assignment of iph

iph is being assigned the same value twice; remove the redundant
first assignment. (Thanks to Nikolay Aleksandrov for pointing out
that the first asssignment should be removed and not the second)

Fixes warning:
net/ipv4/ip_gre.c:265:2: warning: Value stored to 'iph' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>