drivers/net/sfc: Correct code taking the size of a pointer

The function efx_iterate_state contains the code
memcpy(&payload->msg, payload_msg, sizeof(payload_msg));
This is the only use of payload_msg.  The type of payload_msg is
changed from a pointer to an array, so that the result of sizeof really is
the length of the string.

A simplified version of the semantic patch that finds this problem is as
follows: (http://coccinelle.lip6.fr/

)

// <smpl>
@@
expression *x;
expression f;
type T;
@@

*f(...,(T)x,...)
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>