ALSA: hrtimer: make hrtimer_hw const and __initconst

Make this const as it is only used during a copy operation. Also, make
it __initconst as it is only used during the init phase and after this
it is not referenced anywhere.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>