eth: fbnic: Add support to read lane count

We are reporting the lane count in the link settings but the flag is not
set to indicate that the driver supports lanes. Set the flag to report
lane count.

~]# ethtool eth0 | grep Lanes
Lanes: 2

Signed-off-by: Mohsin Bashir <mohsin.bashr@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250924184445.2293325-1-mohsin.bashr@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>