ARM: OMAP2+: onenand: refactor for clarity

Refactor set_async_mode & set_sync_mode functions to
separate out timing calculation & actual configuration
(GPMC & OneNAND side).

Thanks to Jon for his suggestions.

Signed-off-by: Afzal Mohammed <afzal@ti.com>
Reviewed-by: Jon Hunter <jon-hunter@ti.com>