ice: Rename ethtool private flag for lldp

The current flag name of "enable-fw-lldp" is a bit cumbersome.

Change priv-flag name to "fw-lldp-agent" with a value of on or
off.  This is more straight-forward in meaning.

Signed-off-by: Dave Ertman <david.m.ertman@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>