i2c: i801: Refactor mux code since platform_device_unregister() is NULL aware

platform_device_unregister() is NULL-aware and thus doesn't required a
duplication check in i801_del_mux(). Besides that it's also error pointer
aware, and we may drop unneeded assignment in i801_add_mux() followed by
conversion to PTR_ERR_OR_ZERO() for the returned value.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>