mtd: spi-nor: hisi: Avoid generic function names

Using generic names such as get_if_type() is frowned
upon: it suggests a core function (which is not),
and then it makes code navigation harder.

Given drivers are often used as starting point
to write other drivers, generic names tend to spread
like the flu. Cure the problem.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>