i40evf: hide unused variable

On architectures with larger pages, we get a warning about an unused variable:

drivers/net/ethernet/intel/i40evf/i40evf_main.c: In function 'i40evf_configure_rx':
drivers/net/ethernet/intel/i40evf/i40evf_main.c:690:21: error: unused variable 'netdev' [-Werror=unused-variable]

This moves the declaration into the #ifdef to avoid the warning.

Fixes: dab86afd

 ("i40e/i40evf: Change the way we limit the maximum frame size for Rx")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>