i40e: fix condition of WARN_ONCE for stat strings

Commit 9b10df59

 ("i40e: use WARN_ONCE to replace the commented
BUG_ON size check") introduced a warning check to make sure
that the size of the stat strings was always the expected value. This
code accidentally inverted the check of the data pointer. Fix this so
that we accurately count the size of the stats we copied in.

This fixes an erroneous WARN kernel splat that occurs when requesting
ethtool statistics.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Tested-by: Mauro S M Rodrigues <maurosr@linux.vnet.ibm.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>