mtd: devices: elm: fix elm_context_save() and elm_context_restore() functions

These two function's switch case lack the 'break' that make them always
return error.

Signed-off-by: Ted Juan <ted.juan@gmail.com>
Acked-by: Pekon Gupta <pekon@ti.com>
Cc: <stable@vger.kernel.org> # 3.12.x+
Signed-off-by: Brian Norris <computersforpeace@gmail.com>