mtd: rawnand: Fix sphinx syntax

Sphinx doesn't handle expressions in identifier references.

This fixes the following warnings:

./include/linux/mtd/rawnand.h:1184: WARNING: Inline strong start-string without end-string.
./include/linux/mtd/rawnand.h:1186: WARNING: Inline strong start-string without end-string.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>