ice: Use the software based tail when checking for hung Tx ring

Currently in ice_get_tx_pending we try to read a Tx ring's tail. This is
then compared with the software based head (next_to_clean) to determine
if we have pending work. This will never work because reading of the Tx
ring's tail is no longer supported. Fix this by using the software based
tail (next_to_use) to determine if there is pending work.

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>