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

When adding the '{read|write|reg}_proto' fields to 'struct spi_nor', a
colon was missed in the comment for the spi_nor::reg_proto' -- add it.

Fixes: cfc5604c ("mtd: spi-nor: introduce SPI 1-2-2 and SPI 1-4-4 protocols")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>