mtd: bcm47xxsflash: convert kzalloc to avoid invalid access

mtd is just member of bcm47xxsflash, so we should free bcm47xxsflash not
its member. So I use devm_kazlloc instead of kazlloc to avoid it.

* Changelog:
   convert to devm_kzalloc

Signed-off-by: Libo chen <libo.chen@huawei.com>
Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
[Brian: fixed conflicts]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>