ALSA: bebob: Fix prototype of helper function to return negative value

A helper function of ALSA bebob driver returns negative value in a
function which has a prototype to return unsigned value.

This commit fixes it by changing the prototype.

Fixes: eb7b3a05

 ("ALSA: bebob: Add commands and connections/streams management")
Cc: <stable@vger.kernel.org> # v3.16+
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20191026030620.12077-1-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>