ethtool: reduce stack usage

dev_ethtool() is currently using 604 bytes of stack, even with gcc-4.4.2

objdump -d vmlinux | scripts/checkstack.pl
...
0xc04bbc33 dev_ethtool [vmlinux]:			604
...
Adding noinline attributes to selected functions can reduce stack usage.

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