mtd: spi-nor: slightly refactor the spi_nor_setup()

Instead of always using a function pointer (and initializing it to our
default), just call the default function if the flash didn't set its own
one. That will make the call flow easier to follow.

Also mark the parameter as optional now.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20220223134358.1914798-18-michael@walle.cc