mtd: elm: compile suspend/resume only with PM_SLEEP

Fixes:
drivers/mtd/devices/elm.c:480:12: warning: 'elm_suspend' defined but not used [-Wunused-function]
drivers/mtd/devices/elm.c:488:12: warning: 'elm_resume' defined but not used [-Wunused-function]

Signed-off-by: Wolfram Sang <wsa@sang-engineering.com>
Acked-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>