i2c: designware: Synchronize IRQs when unregistering slave client

Make sure interrupt handler i2c_dw_irq_handler_slave() has finished
before clearing the the dev->slave pointer in i2c_dw_unreg_slave().

There is possibility for a race if i2c_dw_irq_handler_slave() is running
on another CPU while clearing the dev->slave pointer.

Reported-by: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Reported-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>