igc: Fix the parameter description for igc_alloc_rx_buffers

The function description for igc_alloc_rx_buffers has not reflected
the function 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>