drivers: misc: ad525x_dpot: Add missing check in dpot_read_spi

The use of dpot_read_r8d8() after checking dpot->uid is similar.
However, we check the return value and return an error code only
in one path, which is odd.

Link: https://lore.kernel.org/r/20210301125057.28819-1-dinghao.liu@zju.edu.cn


Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>