i2c: Remove unnecessary call to irq_find_mapping

irq_create_mapping calls irq_find_mapping internally and will use the
found mapping if one exists, so there is no need to manually call this
from i2c_smbus_host_notify_to_irq.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>