i40e: Separate out DCB capability and enabled flags

Currently if the firmware reports DCB capability the driver enables
I40E_FLAG_DCB_ENABLED flag. When this flag is enabled the driver
inserts a tag when transmitting a packet from the port even if there
are no DCB traffic classes configured at the port.

This patch adds a new flag I40E_FLAG_DCB_CAPABLE that will be set
when the DCB capability is present and the existing flag
I40E_FLAG_DCB_ENABLED will be set only if there are more than one
traffic classes configured at the port.

Change-ID: I24ccbf53ef293db2eba80c8a9772acf729795bd5
Signed-off-by: Neerav Parikh <neerav.parikh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>