sfc: lower stack usage in efx_ethtool_self_test

drivers/net/sfc/ethtool.c: In function ‘efx_ethtool_self_test’:
drivers/net/sfc/ethtool.c:613: warning: the frame size of 1200 bytes
is larger than 1024 bytes

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>