Move the .fds check to ReadRomFile()

The filename is already beeing passed into ReadRomFile(), so just check
it in there instead of requiring IsFds to be passed separately.

cmd/fdslist.go

@@ -32,7 +32,7 @@ func main() {
 }
 
 func run(args *Arguments) error {
-	rom, err := fds.ReadRomFile(args.Input, filepath.Ext(args.Input) == ".fds")
+	rom, err := fds.ReadRomFile(args.Input)
 	if err != nil {
 		return err
 	}

reader.go

@@ -7,18 +7,19 @@ import (
 	"bufio"
 	"bytes"
 	"encoding/binary"
+	"path/filepath"
 
 	"github.com/sigurn/crc16"
 )
 
-func ReadRomFile(filename string, IsFds bool) (*Rom, error) {
+func ReadRomFile(filename string) (*Rom, error) {
 	file, err := os.Open(filename)
 	if err != nil {
 		return nil, err
 	}
 	defer file.Close()
 
-	blocks, err := ReadRomBlocks(file, IsFds)
+	blocks, err := ReadRomBlocks(file, filepath.Ext(filename) == ".fds")
 	if err != nil {
 		return nil, err
 	}