mtd: ssfdc: Improve a size determination in ssfdcr_add_mtd()

Replace the specification of a data structure by a pointer dereference
as the parameter for the operator "sizeof" to make the corresponding size
determination a bit safer according to the Linux coding style convention.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/b7ee7d1b-49a2-41d8-9c8f-3674f1aecc43@web.de