nand_util: treat WITH_YAFFS_OOB as a mode

When specified in the flags argument of nand_write, WITH_YAFFS_OOB causes an
operation which is mutually exclusive with the 'usual' way of writing.

Add a check that client code does not specify WITH_YAFFS_OOB along with any
other flags and add a comment indicating that the WITH_YAFFS_OOB flag should
not be mixed with other flags.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
CC: Scott Wood <scottwood@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>