Revert "drivers/usb: Remove unnecessary return's from void functions" for musb gadget

This is a partial revert of 7f26b3a7533b ("drivers/usb: Remove unnecessary
return's from void functions") as this hunk will vanish in USB tree completely.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>