igc: 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.
_reg_dump method has been partially derived from i210 and have
same typo.

Suggested-by: Gal Hammer <ghammer@redhat.com>
Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>