staging: greybus: use __force when assigning __u8 value to snd_ctl_elem_type_t

staging: greybus: use __force when assigning __u8 value to snd_ctl_elem_type_t

(struct gb_audio_ctl_elem_info*)->type has the type of __u8 so there is no
concern about the byte order. __force is safe to use.

Found by sparse,

$ make C=2 drivers/staging/greybus/
drivers/staging/greybus/audio_topology.c:185:24: warning: cast to restricted snd_ctl_elem_type_t

Suggested-by: Alex Elder <elder@linaro.org>
Signed-off-by: Coiby Xu <coiby.xu@gmail.com>
Link: https://lore.kernel.org/r/20201002233057.74462-3-coiby.xu@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>