enic: add ethtool get_channel support

Add .get_channel to enic_ethtool_ops to enable basic ethtool -l
support to get the current channel configuration.

Note that the driver does not support dynamically changing queue
configuration, so .set_channel is intentionally unused. Instead, users
should use Cisco's hardware management tools (UCSM/IMC) to modify
virtual interface card configuration out of band.

Signed-off-by: Jon Kohler <jon@nutanix.com>
Link: https://patch.msgid.link/20240627202013.2398217-1-jon@nutanix.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>