selftests: drv-net: tso: fix the GRE device name

selftests: drv-net: tso: fix the GRE device name

The device type for IPv4 GRE is "gre" not "ipgre",
unlike for IPv6 which uses "ip6gre".

Not sure how I missed this when writing the test, perhaps
because all HW I have access to is on an IPv6-only network.

Fixes: 0d0f4174f6c8 ("selftests: drv-net: add a simple TSO test")
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250604012031.891242-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>