ALSA: bebob: Use a signed return type for get_formation_index

The return type "unsigned int" was used by the get_formation_index function
despite of the aspect that it will eventually return a negative	error code.
So, change to signed int and get index by reference in the parameters.

Done with the help of Coccinelle.

[Fix the missing braces suggested by Julia Lawall -- tiwai]

Signed-off-by: Lucas Tanure <tanure@linux.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Tested-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>