hw: allwinner-i2c: Fix TWI_CNTR_INT_FLAG on SUN6i SoCs

hw: allwinner-i2c: Fix TWI_CNTR_INT_FLAG on SUN6i SoCs

TWI_CNTR_INT_FLAG is W1C(write 1 to clear and write 0 has non-effect)
register on SUN6i based SoCs, we should lower interrupt when the guest
set this bit.

The linux kernel will hang in irq handler(mv64xxx_i2c_intr) if no
device connected on the i2c bus, next is the trace log:

allwinner_i2c_write write   CNTR(0x0c): 0xc4 A_ACK BUS_EN INT_EN
allwinner_i2c_write write   CNTR(0x0c): 0xcc A_ACK INT_FLAG BUS_EN INT_EN
allwinner_i2c_read  read    CNTR(0x0c): 0xcc A_ACK INT_FLAG BUS_EN INT_EN
allwinner_i2c_read  read    STAT(0x10): 0x20 STAT_M_ADDR_WR_NACK
allwinner_i2c_write write   CNTR(0x0c): 0x54 A_ACK M_STP BUS_EN
allwinner_i2c_write write   CNTR(0x0c): 0x4c A_ACK INT_FLAG BUS_EN
allwinner_i2c_read  read    CNTR(0x0c): 0x4c A_ACK INT_FLAG BUS_EN
allwinner_i2c_read  read    STAT(0x10): 0xf8 STAT_IDLE
allwinner_i2c_write write   CNTR(0x0c): 0x54 A_ACK M_STP BUS_EN
allwinner_i2c_write write   CNTR(0x0c): 0x4c A_ACK INT_FLAG BUS_EN
allwinner_i2c_read  read    CNTR(0x0c): 0x4c A_ACK INT_FLAG BUS_EN
allwinner_i2c_read  read    STAT(0x10): 0xf8 STAT_IDLE
...

Fix it.

Signed-off-by: qianfan Zhao <qianfanguijin@163.com>
Reviewed-by: Strahinja Jankovic <strahinja.p.jankovic@gmail.com>
Tested-by: Strahinja Jankovic <strahinja.p.jankovic@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>