ALSA: usb-audio: Fix invalid NULL check in snd_emuusb_set_samplerate()

The quirk function snd_emuusb_set_samplerate() has a NULL check for
the mixer element, but this is useless in the current code.  It used
to be a check against mixer->id_elems[unitid] but it was changed later
to the value after mixer_eleme_list_to_info() which is always non-NULL
due to the container_of() usage.

This patch fixes the check before the conversion.

While we're at it, correct a typo in the comment in the function,
too.

Fixes: 8c558076 ("ALSA: usb-audio: Clean up mixer element list traverse")
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>