i40e: Use signed variable

The counter variable in i40e_clean_tx_irq starts out negative and climbs
to 0. So it should not be defined as a u16. This was working by accident
due to the fact the u16 overflows and underflows predictably.

Replace the u16 with int, which is signed and can handle the negativity.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>