mtd: parsers: bcm63xx: simplify CFE detection

Instead of trying to parse CFE version string, which is customized by some
vendors, let's just check that "CFE1" was passed on argument 3.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200615091740.2958303-1-noltari@gmail.com