igc: Fix parameter descriptions for a several functions

igc_watchdog, igc_set_interrupt_capability, igc_init_interrupt_scheme,
__igc_open and __igc_close parameter descriptions has not reflected
functions meaning. Add meaningful description.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>