USB: serial: mos7840: fix another NULL-deref at open

Fix another NULL-pointer dereference at open should a malicious device
lack an interrupt-in endpoint.

Note that the driver has a broken check for an interrupt-in endpoint
which means that an interrupt URB has never even been submitted.

Fixes: 3f542974 ("USB: Moschip 7840 USB-Serial Driver")
Cc: stable <stable@vger.kernel.org>	# v2.6.19: 5c75633e
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>