UBI: drop redundant "UBI error" string

The ubi_err() macro automatically prefixes "UBI error" before the message.
By also using it here, we get a log like so:
	UBI error: ubi_init: UBI error: cannot initialize UBI, error -19

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>