i2c: designware: Simplify slave interrupt handler nesting

Interrupt processing code in i2c-designware-slave.c is bit more readable
if not divided into another subroutine. Also explicit IRQ_NONE and
IRQ_HANDLED return values are more obvious.

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>