i2c: Make i2cdev_notifier_call static

The function i2cdev_notifier_call is used only in i2c-dev file
making it static.
Also removes the following sparse warning

drivers/i2c/i2c-dev.c:582:5: warning: symbol 'i2cdev_notifier_call'
was not declared. Should it be static?

Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>