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

Fix NULL-pointer dereference in open() should the device lack the
expected endpoints:

Unable to handle kernel NULL pointer dereference at virtual address 00000030
...
PC is at mos7840_open+0x88/0x8dc [mos7840]

Note that we continue to treat the interrupt-in endpoint as optional for
now.

Fixes: 3f542974

 ("USB: Moschip 7840 USB-Serial Driver")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>