i3c: dw: use bus mode rather than device reg for conditional tCAS setting

In the clock setup path, we set the hardware DEV_CTRL_I2C_SLAVE_PRESENT
bit on a shared mode bus, then read-back this bit for the conditional
tCAS set.

Instead, just use the bus->mode setting for the conditional test.

While we're at it, add a little comment about why the conditional is
there.
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://lore.kernel.org/r/92a933566f7846708a00ad7f5a16ee8e6ed32d0e.1680156630.git.jk@codeconstruct.com.auSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/i3c/master/dw-i3c-master.c

@@ -538,7 +538,11 @@ static int dw_i3c_clk_cfg(struct dw_i3c_master *master)
 	scl_timing = SCL_I3C_TIMING_HCNT(hcnt) | SCL_I3C_TIMING_LCNT(lcnt);
 	writel(scl_timing, master->regs + SCL_I3C_PP_TIMING);
 
-	if (!(readl(master->regs + DEVICE_CTRL) & DEV_CTRL_I2C_SLAVE_PRESENT))
+	/*
+	 * In pure i3c mode, MST_FREE represents tCAS. In shared mode, this
+	 * will be set up by dw_i2c_clk_cfg as tLOW.
+	 */
+	if (master->base.bus.mode == I3C_BUS_MODE_PURE)
 		writel(BUS_I3C_MST_FREE(lcnt), master->regs + BUS_FREE_TIMING);
 
 	lcnt = max_t(u8,