mtd: nand: omap: Switch to using GPMC-NAND ops for writebuffer empty check

Instead of accessing the gpmc_status register directly start
using the gpmc_nand_ops->nand_writebuffer_empty() helper
to check write buffer empty status.

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