cmd: mii: don't check address for 'device' subcommand
All mii operations require a valid PHY address except the 'device'
command, which expects the PHY name rather than the address.
Signed-off-by: Hector Palacios <hector.palacios@digi.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>