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

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.au
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>