UBI: Ensure that "background thread" operations are really executed

The current U-Boot UBI implementation is copied from Linux. In this
porting the UBI background thread was not handled correctly. Upon write
operations ubi_wl_flush() makes sure, that all queued operations, like
page-erase, are completed. But this is missing for read operations.

This patch now makes sure that such operations (like scrubbing upon
bit-flip errors) are not queued, but executed directly.

Signed-off-by: Stefan Roese <sr@denx.de>