mtd: rawnand: brcmnand: Assign soc as early as possible

In order to key off the brcmnand_probe() code in subsequent changes
depending upon ctrl->soc, assign that variable as early as possible,
instead of much later when we have checked that it is non-NULL.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220107184614.2670254-2-f.fainelli@gmail.com