mtd: nand: brcmnand: Change BUG_ON in brcmnand_send_cmd

Change the BUG_ON() condition in brcmnand_send_cmd() which checks for
the interrupt status "controller ready" bit to a WARN_ON.

There is no good reason to kill the system when this condition occur
because we could have systems which listed the NAND controller as
available (e.g: from Device Tree), but the NAND chip could be
malfunctioning and not responding.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Kamal Dasu <kdasu.kdev@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>