[MTD] Clarify OOB-operation interface comments

Add more comment to OOB I/O interface. Read/write are not
symmetric which is confusing and should be documented.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>