i2c: designware: Simplify master interrupt handler nesting

In my opinion a few lines of spurious interrupt detection code can be
moved to the actual master interrupt handling function i2c_dw_isr()
without hurting readability.

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>