mfd: dln2: Fix sanity checking for endpoints

While the commit 2b8bd606 ("mfd: dln2: More sanity checking for endpoints")
tries to harden the sanity checks it made at the same time a regression,
i.e.  mixed in and out endpoints. Obviously it should have been not tested on
real hardware at that time, but unluckily it didn't happen.

So, fix above mentioned typo and make device being enumerated again.

While here, introduce an enumerator for magic values to prevent similar issue
to happen in the future.

Fixes: 2b8bd606 ("mfd: dln2: More sanity checking for endpoints")
Cc: Oliver Neukum <oneukum@suse.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>