ALSA: usb-line6: constify snd_kcontrol_new strucutre array

In kernel APIs of ALSA control interface, drivers can create a control
element set by a call of snd_ctl_new1() with a template. This template
is known to have const qualifier in general cases.

This commit adds the qualifier to template array, for safer program and
runtime. Application of this change moves the symbol from .data section
to .rodata section.

Cc: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>