i2c: iproc: Make bcm_iproc_i2c_quirks constant

Static structure bcm_iproc_i2c_quirks, of type i2c_adapter_quirks, is
only used when being assigned to constant field quirks of a variable
having type i2c_adapter. Hence make bcm_iproc_i2c_quirks constant as
well to prevent it from unintended modification.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>