ixgbevf: only check Tx queue enablement when debugging

Following a write the VFTXDCTL.ENABLE bit is set only when the Tx queue
is actually enabled, which may not happen during the configure phase even
if we waited for it. Make this check debug only since this is causing
confusion with users who notice the warning in dmesg.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>