i2c: designware: Actually make use of the I2C_DW_COMMON and I2C_DW symbol namespaces

DEFAULT_SYMBOL_NAMESPACE must already be defined when <linux/export.h>
is included. So move the define above the include block.

Fixes: fd57a332 ("i2c: designware: Move exports to I2C_DW namespaces")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>