i2c: fix header file kernel-doc warning

Fix kernel-doc warning in <linux/i2c.h>.

../include/linux/i2c.h:337: warning: Function parameter or member 'init_irq' not described in 'i2c_client'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>