i2c: designware: Use temporary variable for struct device

Use temporary variable for struct device to make code neater.

Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>