rockchip: i2c: fix incorrect STOP flag for the interrupt enable register

rockchip: i2c: fix incorrect STOP flag for the interrupt enable register

I2C_CON_STOP is a flag to be used for the con register, where it is bit
4 to send the STOP condition.

To enable the interrupt the controller sends to tell it's finished
sending the STOP condition, it's the ien register at bit 5.

Let's use the proper offset.

My hunch is that enabling the interrupt is useless as the interrupt
status register is always up-to-date and enabling the interrupt is just
so that the interrupt is available via the GIC. However, U-Boot has no
interrupt support and the logic was working well before this patch. This
is just so people aren't side-tracked when debugging I2C issues on
Rockchip by checking all writes are proper.

Fixes: 3437469985df ("rockchip: Add I2C driver")
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Heiko Schocher <hs@nabladev.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>