igb: fix TDBAL register show incorrect value

Fixed a typo which caused the registers dump function to read the
RDBAL register when printing TDBAL register values.

Signed-off-by: Gal Hammer <ghammer@redhat.com>
Tested-by: David Switzer <david.switzer@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>