Handle when the first card is not card0

While we're still only looking at the first video card, we now handle when the first card is not
card0 but perhaps card1.

lib/amdgpu_fan/cli.rb

@@ -169,6 +169,7 @@ def watch_csv(seconds = 1)
     private
 
     def amdgpu_service
+      # Right now we're only looking at the first card.
       @amdgpu_service ||= AmdgpuFan::Service.new
     end
 

lib/amdgpu_fan/service.rb

@@ -22,8 +22,12 @@ class Service
 
     class Error < StandardError; end
 
-    def initialize(card_num: 0)
-      @card_num = card_num
+    def self.card_numbers
+      Dir["#{BASE_FOLDER}/card*"].filter_map { |path| path.split('/').last.slice(/(?<=card)\d+\Z/) }
+    end
+
+    def initialize(card_num: nil)
+      @card_num = card_num || self.class.card_numbers.first
     end
 
     def busy_percent

spec/lib/amdgpu_fan/service_spec.rb

@@ -12,6 +12,7 @@
   before do
     FileUtils.mkdir_p file_dir
     stub_const "#{described_class}::BASE_FOLDER", base_dir
+    allow(described_class).to receive(:card_numbers).and_return([0])
   end
 
   describe '#busy_percent' do