USB: usbtmc: fix probe error path

USB: usbtmc: fix probe error path

commit 2e47c53503eb9faff42b3cfa144a833344dd1f89 upstream.

Make sure to initialise the return value to avoid having allocation
failures going unnoticed when allocating interrupt-endpoint resources.

This prevents use-after-free or worse when the device is later unbound.

Fixes: dbf3e7f654c0 ("Implement an ioctl to support the USMTMC-USB488 READ_STATUS_BYTE operation.")
Cc: Dave Penkler <dpenkler@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>