Don't reset the configuration if desired config is already active (#6)

Fixes issue #5

AUTHORS

@@ -11,3 +11,4 @@ Ivan Krasin <imkrasin@gmail.com>
 Jirawat I. <nodtem66@gmail.com>
 Thordur Bjornsson <thorduri@secnorth.net>
 Vincent Serpoul <vincent@serpoul.com>
+Josef Filzmaier <josef.filzmaier@gmail.com>

device.go

@@ -128,9 +128,14 @@ func (d *Device) Config(cfgNum int) (*Config, error) {
 		dev:     d,
 		claimed: make(map[int]bool),
 	}
+
+	if activeCfgNum, err := d.ActiveConfigNum(); err != nil {
+		return nil, fmt.Errorf("failed to query active config of the device %s: %v", d, err)
+	} else if cfgNum != activeCfgNum {
 		if err := libusb.setConfig(d.handle, uint8(cfgNum)); err != nil {
 			return nil, fmt.Errorf("failed to set active config %d for the device %s: %v", cfgNum, d, err)
 		}
+	}
 	d.mu.Lock()
 	defer d.mu.Unlock()
 	d.claimed = cfg