drm/i915/intel_i2c: handle zero-length writes

A common method of probing an i2c bus is trying to do a zero-length write.
Handle this case by checking the length first before decrementing it.

This is actually important, since attempting a zero-length write is one
of the ways that i2cdetect and i2c_new_probed_device detect whether
there is device present on the bus with a given address.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_i2c.c

@@ -248,9 +248,10 @@ gmbus_xfer_write(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
 	u32 val, loop;
 
 	val = loop = 0;
-	do {
-		val |= *buf++ << (8 * loop);
-	} while (--len && ++loop < 4);
+	while (len && loop < 4) {
+		val |= *buf++ << (8 * loop++);
+		len -= 1;
+	}
 
 	I915_WRITE(GMBUS3 + reg_offset, val);
 	I915_WRITE(GMBUS1 + reg_offset,