i2c: designware: Don't use internal ___constant_swab32

It looks ___constant_swab32 is an internal byte swap detail. Switch to
swap32() and include header file for it explicitly. Now it comes
implicitly via linux/i2c.h include.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>