usb: gadget: dummy_hcd: fix rescan logic for transfer

transfer() schedules a rescan for transfers larger than
maxpacket, which is wrong for transfers that are multiples
of maxpacket.

Rewrite to fix and clarify packet multiple / remainder
transfer logic.

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