igb: minor ethool regdump amendment

This patch has no functional impact and it is just a preparation
for the following patch. It removes an early return from the
'igb_get_regs()' function by moving the 82576-only registers
dump into an "if" block. With this preparation, we can dump more
non-82576 registers at the end of this function.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>