usb: gadget: f_sourcesink.c: correct a copy-paste misnomer

acm was the first function to be converted and it seems that its
code served as a base for converting f_sourcesink to the new function
interface. source_sink has nothing to do with acm, though.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>