ixgbe: Return 64 bit stats values

The code was ignoring higher 32 bits of stats registers. This patch
correctly fills out 64 bit value in two 32 bit words.

Signed-off-by: Preethi Banala <preethi.banala@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>