doc:networking: Update comment for dev_id field in netdevice.h

This patch updates the comment for 'dev_id' field in
'include/linux/netdevice.h' to reflect the intended
usage of 'dev_id'.

References: http://marc.info/?l=linux-netdev&m=136992115300526&w=2
References: http://marc.info/?l=linux-netdev&m=137062569014612&w=2



Signed-off-by: Narendra K <narendra_k@dell.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>