mtd: spi-nor: issi: unify function names

mtd: spi-nor: issi: unify function names

To avoid name clashes unify all the function and static object names and
use one of the following prefixes which should be sufficiently unique:
 - <vendor>_nor_
 - <flash_family>_nor_
 - <flash_part>_

There are no functional changes.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Acked-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20220223134358.1914798-10-michael@walle.cc