ixgbe: Change register variable to unsigned

I noticed this variable used for register reads wasn't an unsigned
so this patch corrects that.  I don't believe this was causing any
issue as is but this is more consistent with the rest of the driver.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>