omap_gpmc: change nandecc command

With uppcoming BCH support on OMAP devices we need to decide between differnt
algorithms when switching the ECC engine.  Currently we support 1-bit hammign
and 8-bit BCH on HW backend.

In order to switch between differnet ECC algorithms we need to change the
interface of omap_nand_switch_ecc() also.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: Tom Rini <trini@ti.com>
Cc: Thomas Weber <thomas.weber.linux@googlemail.com>