mtd: spi-nor: fix kernel-doc for spi_nor::mtd

When embedding 'struct mtd_info' within 'struct spi_nor', the kernel-doc
comment was forgotten. Fix it by dropping the "pointer to" part from the
comment.

Fixes: 19763671 ("mtd: spi-nor: embed struct mtd_info within struct spi_nor")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>