mtd: spi-nor: Fix multiple typos

There are a few typos in comments in the SPI NOR framework; fix them.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20201130152416.1283972-1-j.neuschaefer@gmx.net