UBI: clean-up printing helpers

Let's prefix UBI messages with 'ubiX' instead of 'UBI-X' - this is more
consistent with the way we name UBI devices.

Also, commit "32608703 UBI: Extend UBI layer debug/messaging capabilities"
added the function name print to 'ubi_msg()' - lets revert this change, since
these messages are supposed to be just informative messages, and not debugging
messages.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>