bng_en: add ethtool link settings, get_link, and nway_reset

bng_en: add ethtool link settings, get_link, and nway_reset

Add get/set_link_ksettings, get_link, and nway_reset support.
Report supported, advertised, and link-partner speeds across NRZ,
PAM4, and PAM4-112 signaling modes. Enable lane count reporting.

Signed-off-by: Bhargava Marreddy <bhargava.marreddy@broadcom.com>
Reviewed-by: Vikas Gupta <vikas.gupta@broadcom.com>
Reviewed-by: Rajashekar Hudumula <rajashekar.hudumula@broadcom.com>
Reviewed-by: Ajit Kumar Khaparde <ajit.khaparde@broadcom.com>
Signed-off-by: Vikas Gupta <vikas.gupta@broadcom.com>
Link: https://patch.msgid.link/20260406180420.279470-4-bhargava.marreddy@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>