i2c: designware: make use of devm_gpiod_get_optional

There is a semantical change: if devm_gpiod_get_optional returns -ENOSYS
this is passed as error to the caller. This effectively reverts commit
d1fa7452 ("i2c: designware: Consider SCL GPIO optional") which
shouldn't be necessary any more since gpiod_get_optional doesn't return
-ENOSYS any more with GPIOLIB=n.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>