drm/i915/intel_i2c: handle zero-length reads

A common method of probing an i2c bus is trying to do a zero-length read.
Handle this case by checking the length first waiting for data to be read.

This is actually important, since attempting a zero-length read 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.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=48269
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_i2c.c

@@ -217,7 +217,7 @@ gmbus_xfer_read(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
 		   (len << GMBUS_BYTE_COUNT_SHIFT) |
 		   (msg->addr << GMBUS_SLAVE_ADDR_SHIFT) |
 		   GMBUS_SLAVE_READ | GMBUS_SW_RDY);
-	do {
+	while (len) {
 		int ret;
 		u32 val, loop = 0;
 		u32 gmbus2;
@@ -235,7 +235,7 @@ gmbus_xfer_read(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
 			*buf++ = val & 0xff;
 			val >>= 8;
 		} while (--len && ++loop < 4);
-	} while (len);
+	}
 
 	return 0;
 }