mtd: Update the function name to 'rfree'

Since commit 8d38a845 ("mtd: Rename free() to rfree()")
the function has been renamed to rfree(), so update the description
inside the mtd_oob_region structure as well.

Fixes: 8d38a845 ("mtd: Rename free() to rfree()")
Reported-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>