mtd: spi-nor: Separate preparation and locking

While this operation will remain a single function call in the end,
let's extract the logic of the [un]prepare calls within their own static
helper. We will soon add new flavors of the *_[un]prepare_and_[un]lock()
helpers, having the preparation logic outside will save us from duplicating
code over and over again.

There is no functional change.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20230328154105.448540-5-miquel.raynal@bootlin.com


Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>