sfc: Fix memcpy() with const destination compiler warning.

drivers/net/ethernet/sfc/selftest.c: In function ‘efx_iterate_state’:
drivers/net/ethernet/sfc/selftest.c:388:9: warning: passing argument 1 of ‘memcpy’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-array-qualifiers]

This is because the msg[] member of struct efx_loopback_payload
is marked as 'const'.  Remove that.

Signed-off-by: David S. Miller <davem@davemloft.net>