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

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: eb7b3a056cd8 ("ALSA: bebob: Add commands and connections/streams management")
Cc: <[email protected]> # v3.16+
Signed-off-by: Takashi Sakamoto <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Takashi Iwai <[email protected]>