ALSA: line6: Remove superfluous NULL checks in each driver

The interface and driver objects are always set when callbacks are
called.  Drop such superfluous NULL checks in init and disconnect
calls of each driver.

Tested-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --cc sound/usb/line6/pod.c
Simple merge

diff --cc sound/usb/line6/podhd.c
Simple merge

diff --cc sound/usb/line6/toneport.c
Simple merge

diff --cc sound/usb/line6/variax.c
Simple merge