mtd: bcm47xxsflash: use platform_(set|get)_drvdata

mtd: bcm47xxsflash: use platform_(set|get)_drvdata

We have generic place & helpers for storing platform driver data so
there is no reason for using custom priv pointer.

This allows cleaning up struct bcma_sflash from unneeded fields.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Kalle Valo <kvalo@codeaurora.org>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>