mtd: nand: pxa3xx: Allow to set/clear the 'spare enable' field

Some commands (such as the ONFI parameter page read) need to
clear the 'spare enable' bit. This commit allows to set/clear
depending on the prepared command, instead of having it always
set.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>