i2c: Drop unneeded check for of_node

of_find_property() will return NULL if of_node is NULL,
thus of_irq_get_by_name() returns -EINVAL which we ignore,
so no need to double check.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>