igc: Get rid of igc_max_channels()

The local function igc_max_channels() is a pointless wrapper around
igc_get_max_rss_queues(). This patch removes it and updates the callers
accordingly. It also does some cleanup on igc_get_max_rss_queues().

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>