i2c: designware_i2c: Restore enable state after set speed

i2c: designware_i2c: Restore enable state after set speed

Before calling __dw_i2c_set_bus_speed(),
the I2C could already be set as ether enable or disable,
we should restore the original setting instead of enable i2c anyway.

This patch fix a bug happened in init function:
    __dw_i2c_init(){
            /* Disable i2c */
            ...
            __dw_i2c_set_bus_speed(i2c_base, NULL, speed);
            writel(slaveaddr, &i2c_base->ic_sar);
            /* Enable i2c */
    }
In this case, enable i2c inside __dw_i2c_set_bus_speed() function
will cause ic_sar write fail.

Signed-off-by: Jun Chen <ptchentw@gmail.com>