ALSA: usb-audio: Avoid superfluous usb_set_interface() calls

This is a preliminary change for the upcoming quirk implementation.

Currently USB-audio driver tries to call usb_set_interface() whenever
the format change with interface/altset modification happens.  In this
patch, the check is replaced with the comparison of cur_altsetting and
the targeted altsetting pointer, so that the driver may skip the
unnecessary function calls.

Signed-off-by: Takashi Iwai <tiwai@suse.de>