i2c: dev: make i2c_dev_class a static const structure

Now that the driver core allows for struct class to be in read-only
memory, move the i2c_dev_class structure to be declared at build time
placing it into read-only memory, instead of having to be dynamically
allocated at boot time.

Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>