UBI: clean-up terminology for self-checks in io.c

We have the "sefl-check" feature in UBI, but for historical reasons many
corresponding functions and commentaries in the code use term "paranoid check"
instead. Let's clean this up and use "self-check" everywhere.

This patch renames functions, amends comments and messages. It touches only the
io.c file.

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