USB: serial: mxuport: add endpoint sanity check

Add an explicit sanity check to make sure we have the expected
endpoints. This will provide a descriptive error message in case an
expected endpoint is missing when probing.

Note that the driver already gracefully fails to probe (albeit with a
less descriptive error message) if a bulk-in endpoint is missing, and an
attempt to write to a port whose device lack a bulk-out endpoint would
fail with -ENODEV.

Signed-off-by: Johan Hovold <johan@kernel.org>