mlxsw: core: Use variable timeout for EMAD retries

[ Upstream commit 1f492eab67bced119a0ac7db75ef2047e29a30c6 ]

The driver sends Ethernet Management Datagram (EMAD) packets to the
device for configuration purposes and waits for up to 200ms for a reply.
A request is retried up to 5 times.

When the system is under heavy load, replies are not always processed in
time and EMAD transactions fail.

Make the process more robust to such delays by using exponential
backoff. First wait for up to 200ms, then retransmit and wait for up to
400ms and so on.

Fixes: caf7297e ("mlxsw: core: Introduce support for asynchronous EMAD register access")
Reported-by: Denis Yulevich <denisyu@nvidia.com>
Tested-by: Denis Yulevich <denisyu@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>