ixgbe: Prevent u8 wrapping of ITR value to something less than 10us

There were a couple cases where the ITR value generated via the adaptive
ITR scheme could exceed 126. This resulted in the value becoming either 0
or something less than 10. Switching back and forth between a value less
than 10 and a value greater than 10 can cause issues as certain hardware
features such as RSC to not function well when the ITR value has dropped
that low.

CC: stable@vger.kernel.org
Fixes: b4ded832 ("ixgbe: Update adaptive ITR algorithm")
Reported-by: Gregg Leventhal <gleventhal@janestreet.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>