mtd: brcmnand: Fix pointer type-cast in brcmnand_write()

We should always type-cast pointer to "long" or "unsigned long"
because size of pointer is same as machine word size. This will
avoid pointer type-cast issues on both 32bit and 64bit systems.

This patch fixes pointer type-cast issue in brcmnand_write()
as-per above info.

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Reviewed-by: Vikram Prakash <vikramp@broadcom.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>