i2c: core: Tidy up handling of init_irq

Only set init_irq during i2c_device_new and only handle client->irq on
the probe/remove paths.

Suggested-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>