ixgbevf: fix size of queue stats length

IXGBEVF_QUEUE_STATS_LEN is based on ixgebvf_stats, not ixgbe_stats.

This change fixes a bug where ethtool -S displayed some empty fields.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>