mtd: bcm47xxsflash: convert to module_platform_driver instead of init/exit

convert to module_platform_driver instead of init/exit

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