mlxsw: core: Avoid unnecessary EMAD buffer copy

mlxsw_emad_transmit() takes care of sending EMAD transactions to the
device. Since these transactions can time out, the driver performs up to
5 retransmissions, each time copying the skb with the original request.

The data of the skb does not change throughout the process, so there is
no need to copy it each time. Instead, only the skb itself can be
copied. Therefore, use skb_clone() instead of skb_copy().

This reduces the latency of the function by about 16%.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>