gve: Decouple header split from RX buffer length

gve: Decouple header split from RX buffer length

Previously, enabling header split via `gve_set_hsplit_config` also
implicitly changed the RX buffer length to 4K (if supported by the
device). This coupled two settings that should be orthogonal; this patch
removes that side effect.

After this change, `gve_set_hsplit_config` only toggles the header
split configuration. The RX buffer length is no longer affected and
must be configured independently.

Signed-off-by: Ankit Garg <nktgrg@google.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Reviewed-by: Jordan Rhee <jordanrhee@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Joshua Washington <joshwash@google.com>
Link: https://patch.msgid.link/20251106192746.243525-2-joshwash@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>