gve: Cache link_speed value from device

gve: Cache link_speed value from device

The link speed is never changed for the uptime of a VM, and the current
implementation sends an admin queue command for each call. Admin queue
command invocations have nontrivial overhead (e.g., VM exits), which can
be disruptive to users if triggered frequently. Our telemetry data shows
that there are VMs that make frequent calls to this admin queue command.
Caching the result of the original admin queue command would eliminate
the need to send multiple admin queue commands on subsequent calls to
retrieve link speed.

Fixes: 7e074d5a76ca ("gve: Enable Link Speed Reporting in the driver.")
Signed-off-by: Joshua Washington <joshwash@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230321172332.91678-1-joshwash@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>