usb: raw-gadget: Fix copy_to/from_user() checks

usb: raw-gadget: Fix copy_to/from_user() checks

The copy_to/from_user() functions return the number of bytes remaining
but we want to return negative error codes.  I changed a couple checks
in raw_ioctl_ep_read() and raw_ioctl_ep0_read() to show that we still
we returning zero on error.

Fixes: f2c2e717642c ("usb: gadget: add raw-gadget interface")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Andrey Konovalov <andreyknvl@google.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>