ixgbe: Move all values that deal with count, next_to_use, next_to_clean to u16

This change updates all values dealing with count, next_to_use, and
next_to_clean so that they stay u16 values.  The advantage of this is that
there is no re-casting of type during the propagation through the stack.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>