disk: part: Don't show redundant error message
Underlying API should already print some meaningful error message, so
this one is just brings more noise. E.g. we can see log like this:
MMC: no card present
** Bad device mmc 0 **
Obviously, second error message is unwanted. Let's only print it in case
when DEBUG is defined to keep log short and clear.
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>