mtd: rawnand: brcmnand: Avoid pdev in brcmnand_init_cs()

In preparation for encapsulating more of what the loop calling
brcmnand_init_cs() does, avoid using platform_device when it is the
device behind platform_device that we are using for printing errors.

No functional changes introduced.

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/20220107184614.2670254-4-f.fainelli@gmail.com