usb: gadget: dummy_hcd: in transfer(), return data sent, not limit

dummy_timer uses transfer() to update transfer limit. However,
limit passed to dummy_timer changes depending on transfer type,
so the actual limit is overwritten.

This can cause unpredictably slow / fast bulk transfers when
coupled with control / interrupt transfers.

Fix by returning actual amount of data sent in transfer() and
substracting from total.

Signed-off-by: Igor Kotrasinski <i.kotrasinsk@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>