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

When introducing 'struct spi_nor', a number of issues was added in its
kernel-doc comment:

- double article in the heading kernel-doc comment;
- "point" instead of "pointer" for the 'mtd' and 'dev' fields;
- "a" articles instead of "an" for the 'dev' field;
- acronyms in the lower case for the 'dev' field;
- missing "pointer to" for the 'priv' field.

Fix all of those at once...

Fixes: 6e602ef7 ("mtd: spi-nor: add the basic data structures")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>