i40evf: fix compiler warning of unused variable

Compiler complained of an unused variable, which the driver was just
using to store the result of a rd32 which is used to clear a register
unconditionally.  Just drop the unused variable and re-use one.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>