UBI: block: Make ubiblock_resize return something

Currently, ubiblock_resize() can fail if the device is not found
in the list. This commit changes the return type, so the function can
return something meaningful on error paths.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>