usb: dwc3: ep0: preparation for handling non maxpacket aligned transfers > 512

No functional change. This is in preparation for handling non maxpacket
aligned transfers greater than bounce buffer size. This is basically to
avoid code duplication when using chained TRB transfers to handle
non maxpacket aligned transfers greater than bounce buffer size.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>