etherdevice: fix comments

Fix some minor problems in comments of etherdevice.h
 * Warning is out dated, file hasn't moved or disappeared in many years and
    is unlikely to do so soon.
 * Capitalize Ethernet consistently since it is a proper name
 * Fix descriptive comment of padding
 * Spelling and grammar fix for alignment comment

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>