nand_spl: Fix cmd_ctrl usage in nand_boot.c.

nand_spl: Fix cmd_ctrl usage in nand_boot.c.

When adding large page NAND support to this file, I had a misunderstanding
about the exact semantics of NAND_CTRL_CHANGE (which isn't documented
anywhere I can find) -- it is apparently just a hint to drivers,
which aren't required to preserve the old value for subsequent
non-"change" invocations.

This change makes nand_boot.c no longer assume this.  Note that this
happened to work by chance with some NAND drivers, which don't preserve
the value, but treat 0 equivalently to NAND_CTRL_ALE.

I don't have hardware to test this, so any testing is appreciated.

Signed-off-by: Scott Wood <scottwood@freescale.com>