mtd: rawnand: brcmnand: Allow working without interrupts

The BCMA devices include the brcmnand controller but they do not wire up
any interrupt line, allow the main interrupt to be optional and update
the completion path to also check for the lack of an interrupt line.

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/20220107184614.2670254-6-f.fainelli@gmail.com