virtchnl: Use the BIT() macro for capability/offload flags

Currently raw hex values are used to define specific bits for each
capability/offload in virtchnl.h. Using raw hex values makes it
unclear which bits are used/available. Fix this by using the BIT()
macro so it's immediately obvious which bits are used/available.

Also, move the VIRTCHNL_VF_CAP_ADV_LINK_SPEED define in the correct
place to line up with the other bit values and add a comment for its
purpose.

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Tested-by: Tony Brelinski <tony.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>