usb: gadget: f_sourcesink: fix function params handling

Move function parameters to struct f_sourcesink to make them per instance
instead of having them as global variables. Since we can have multiple
instances of USB function we also want to have separate set of parameters
for each instance.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>