memory: omap-gpmc: Prevent GPMC_STATUS from being accessed via gpmc_regs

GPMC_STATUS register is private to the GPMC module and must not be
accessed directly by NAND driver through the gpmc_regs.

They must use gpmc_omap_get_nand_ops() instead.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>