i2c: designware: Remove ->disable() callback

Commit 90312351 ("i2c: designware: MASTER mode as separated driver")
introduced ->disable() callback but there is no real use for it. Both
i2c-designware-master.c and i2c-designware-slave.c set it to the same
i2c_dw_disable() and scope is inside the same kernel module.

That said, replace the callback by explicitly calling the i2c_dw_disable().

Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>