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

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

Fixes: 068fbff4f860 ("usb: raw-gadget: Fix copy_to/from_user() checks")
Fixes: f2c2e717642c ("usb: gadget: add raw-gadget interface")
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>
Bug: 147413187
(cherry picked from commit 6e507644209b4a043363e165855a3a3050c4d40e)
Change-Id: I16f2ab1d881a2313bd1ec6b77aa0229fe08fe25b
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>