mtd: nand: pxa3xx: Add __maybe_unused keyword to enable_int()

Now that we have added ARCH_HAS_DMA conditional the function
enable_int() may be unused. Declare it as __maybe_unused,
in order to remove the following warning, when the function is not used:

drivers/mtd/nand//pxa3xx_nand.c:343:24: warning: 'enable_int' defined
but not used [-Wunused-function]

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>