drm/bridge: analogix_dp: return error if transfer none byte

Reference from drm_dp_aux description (about transfer):
Upon success, the implementation should return the number of payload bytes
that were transferred, or a negative error-code on failure. Helpers
propagate errors from the .transfer() function, with the exception of
the -EBUSY error, which causes a transaction to be retried. On a short,
helpers will return -EPROTO to make it simpler to check for failure.

The analogix_dp_transfer will return num_transferred, but if there is none
byte been transferred, the return value will be 0, which means success, we
should return error-code if transfer none byte.

for (retry = 0; retry < 32; retry++) {
err = aux->transfer(aux, &msg);
if (err < 0) {
if (err == -EBUSY)
continue;

goto unlock;
}
}

Cc: zain wang <[e-mail]>
Reviewed-by: Sean Paul <[e-mail]>
Signed-off-by: Jianqun Xu <[e-mail]>
Signed-off-by: Archit Taneja <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c

···
 		 (msg->request & ~DP_AUX_I2C_MOT) == DP_AUX_NATIVE_READ)
 		msg->reply = DP_AUX_NATIVE_REPLY_ACK;
 
-	return num_transferred;
+	return num_transferred > 0 ? num_transferred : -EBUSY;
 }