usb: raw-gadget: fix return value of ep read ioctls

They must return the number of bytes transferred during the data stage.

Fixes: 068fbff4 ("usb: raw-gadget: Fix copy_to/from_user() checks")
Fixes: f2c2e717 ("usb: gadget: add raw-gadget interface")
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>