igb: move PEROUT and EXTTS isr logic to separate functions

Remove code duplication in the tsync interrupt handler function by moving
this logic to separate functions. This keeps the interrupt handler readable
and allows the new functions to be extended for adapter types other than
i210.

Signed-off-by: Ruud Bos <kernel.hbk@gmail.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>