ALSA: control: remove compilation warning on 32-bit

This was introduced by 'ALSA: control: add support for ENUMERATED user
space controls' which adds a u64 variable that gets cast to a pointer:

sound/core/control.c: In function 'snd_ctl_elem_init_enum_names':
sound/core/control.c:1089: warning: cast to pointer from integer of different size

Cast to uintptr_t before casting to pointer to avoid the warning.

Signed-off-by: Olof Johansson <olof@lixom.net>
[cl: replace long with uintptr_t]
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>