i2c: designware: Define software status flags with BIT()

Define software status flags with a BIT() macro. While at it remove
STATUS_IDLE and replace its use with zero initialization and status
flags clearing with a mask.

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